Improve performance of viewing individual issues

This MR does two things:

1. `Issue#related_branches` no longer performs Git operations that aren't needed
2. The output of `Repository#exists?` is now cached and flushed properly

Combined these two changes should further cut down the amount of Git operations performed when viewing individual issues (and possibly other pages).

See merge request !3296

Ability to move issue to another project

Tasks:

- [x] Create scaffold of service that will move issue to another project.
- [x] Close old issue, add system note about moving issue to a new project.
- [x] Create a new issue, add system note about issue being moved from old project.
- [x] Check if issue can be moved to another project before executing service
- [x] Check permissions when moving an issue (`:admin_issue` ability)
- [x] Display select box for a new project when editing an issue
- [x] Show only projects that issue can be moved into in that select box
- [x] Add project select handler, helper and some permission filters to it
- [x] Preserve as much information as possible, including author
- [x] Prepare mechanisms that unfolds local references in issue description
- [x] Rewrite issue description with references unfolding and add some specs for it
- [x] Rewrite all system notes and comments attached to issue that is being moved
- [x] Update `Label` so that is was able to create cross reference labels (separate MR)
- [x] Add notifications about moving issue to another project
- [x] Display confirmation alert/message when issue move has been requested
- [x] Make it possible to undo selecting project where issue will be moved to
- [x] Add column to issue, that will indicate if it has been moved to another project
- [x] Do not allow to move issue that has been already moved
- [x] Write top-to-bottom feature spec in RSpec instead of Spinach

UI:

![issue_move_ui](/uploads/b3c6b563362c1fded9082cc0f51e5a74/issue_move_ui.png)

![issue_move_tooltip](/uploads/2ab913b06f52df1cafde9abe89bd9cb8/issue_move_tooltip.png)

Closes #3024

See merge request !2831

Use gitlab-workhorse 0.7.1

A small extra defense against interrupted git raw responses. Nice to
have in 8.6 because we introduce /raw/ caching; it would be
unfortunate if incorrect (truncated) responses got cached.

See merge request !3330

Fix diff image view modes (2-up, swipe, onion skin) not working

Example test case: https://gitlab.com/mrtzcspr/test/commit/5d983226b1308d98ef3d8919b978ad49f34496b8

Closes #13856

See merge request !3316

Use the configured Kaminari "per page" default

Configured in [`config/initializers/kaminari.rb`](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/initializers/kaminari_config.rb).

See merge request !3320

Backport premailer and diff changes from EE

Backport part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/151

See merge request !3318

These tests would check if the "This project is empty" banner would
contain SSH clone URLs. Oddly enough this should have never passed (as
far as I can tell) as SSH clone URLs in this banner are _only_ displayed
if the current user has at least 1 SSH key attached. Since the tests
never seem to create any they never should have passed, yet somehow they
did. To solve this the Spinach tests in question now ensure at least 1
SSH key is present.

Upgrade Rubocop from 0.35.1 to 0.38.0

See [Rubocop's Releases page](https://github.com/bbatsov/rubocop/releases) for more info on what changed.

Descriptions/StyleGuides removed as discussed in #14233.

Changes:
- Enable DisplayCopNames for lint output.
- Default behavior for `Alias` changed, set to enforce `prefer_alias_method`.
- Enabling Rails cops changed to new syntax.
- Remove StyleGuides and move Descriptions to comments.
- Add missing cops.
- Add TODOs for cops that should be enabled in the future.
- Set TargetRubyVersion to 2.1.

See merge request !3197

Discussed in #14233.

See [their releases](https://github.com/bbatsov/rubocop/releases) for
more info.

Changes:
- Enable DisplayCopNames for lint output.
- Default behavior for `Alias` changed, set to enforce `prefer_alias_method`.
- Enabling Rails cops changed to new syntax.
- Remove StyleGuides and move Descriptions to comments.
- Add missing cops.
- Add TODOs for cops that should be enabled in the future.
- Set TargetRubyVersion to 2.1.

Fix a few remaining uses of `0px`.

See #14299 for more information.

cc: @jschatz1 

See merge request !3310

Make HTTP(s) label consistent on clone bar

Sites that use http:// for the external_url should always display HTTP on
the clone bar. Similarly, sites that use https:// should show HTTPS.

Currently, the inconsistency looks like:

![http-magic](/uploads/945fda580f7ba5aee36cc572b49baf2b/http-magic.gif)

Also restores the tooltips that vanished in 7ba4482f:

![image](/uploads/b5a7b3e68b4992a447fcaa51915e866c/image.png)


See merge request !3319

Fix spacing below edit note form

Also changed to allow the CSS to decide what should be hidden when editing

Thanks @JobV for having eagle eyes 😜 

See merge request !3306

Expand the issuable sidebar when coming back from the Changes tab

Unless the user explicitly requested the collapsed sidebar.

Fixes: #14353

See merge request !3256

Fixed issue with filtered data persisting when dropdown closed

![labels](/uploads/4b9c902add76263d8488cc34ef49f142/labels.gif)

Closes #14401

See merge request !3302

* master:
  Fix bug where wrong commit ID was being used in a merge request diff to show old image
  Remove CHANGELOG item that was added during merge resolution
  Improve the "easy WIP & un-WIP from link" feature
  Fix specs
  \#to_branch_name now uses the iid as postfix
  Add label description in tooltip to labels in issue index and sidebar
  Easily (un)mark merge request as WIP using link
  Use specialized system notes when MR is (un)marked as WIP
  another attempt to fix oauth issue
  attempting to fix omniauth problem

Conflicts:
	app/assets/javascripts/issuable_form.js.coffee

Sites that use http:// for the external_url should always display HTTP on
the clone bar. Similarly, sites that use https:// should show HTTPS.

Fix bug where wrong commit ID was being used in a merge request diff to show old image

Closes #3769

Closes #14327

See merge request !3300

Similar to ad90dba5185e30883d5ad6008c166b0df0108ebf we always have a
repository in the real world, so let's also create one in our Spinach
tests.

In the real world a project always has a repository. This fact allows
code such as Issue#related_branches to work without explicitly checking
if a repository exists.

This caches the output of Repository#exists? in Redis while making sure
it's flushed properly when creating new repositories, deleting them,
etc.

For the ProjectWiki tests to work I had to make ProjectWiki#create_repo!
public as testing private methods in RSpec is a bit of a pain.

Requesting the branch names of a repository works even when it's empty,
thus there's no need to explicitly check for an empty repository.
Removing this check cuts down the amount of Git operations which in turn
cuts down request timings a bit. The regular expression used to compare
branches was also moved out of the loop so it's created only once.

Closes #3769

Closes #14327

Easily (un)mark merge request as WIP using link

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/3768 and https://gitlab.com/gitlab-org/gitlab-ce/issues/3516

## Link to add `WIP` prefix (underline is visible because of hover)
![wipless_title](/uploads/72a6f7119ba9d8043ca8329641e97c3b/wipless_title.png)

## Link to remove `WIP` prefix
![wip_title](/uploads/8620ad65da9ef620b180603520fead55/wip_title.png)

## System note after WIP is added
![wip_sysnote](/uploads/2de073b75e854d2c9e243eb8b5d5c259/wip_sysnote.png)

## Widget with link to remove WIP
![wip_widget](/uploads/cf83ea93743c4c26d9df759c17cb9d7b/wip_widget.png)

## Flash after WIP is removed
![wip_flash](/uploads/27b7240cd5d7ceeb8b7b477abd94d7ff/wip_flash.png)

## System note after WIP is removed
![wipless_sysnote](/uploads/c0d3368abdf21a2f253532a9a9594d90/wipless_sysnote.png)

## Widget when current user cannot remove the WIP prefix
![wip_widget_unauthorized](/uploads/174ccf1674be86dc81c3078fe297acb7/wip_widget_unauthorized.png)

cc @creamzy 

See merge request !3006

# Conflicts:
#	Gemfile.lock

attempting to fix omniauth problem

Attempt to fix https://gitlab.com/gitlab-org/gitlab-ce/issues/3361

See merge request !2876

Add label description in tooltip to labels in issue index and sidebar

The tooltip can get quite big, but it’s so useful to inform people about what these labels mean
new

![Screen_Shot_2016-03-08_at_11.48.28](/uploads/894baaaa1f789c3a2069a924d541e64b/Screen_Shot_2016-03-08_at_11.48.28.png)

![Screen_Shot_2016-03-08_at_11.48.20](/uploads/efe4fa0960054c4071b8f6bf83024403/Screen_Shot_2016-03-08_at_11.48.20.png)

@JobV writes on Slack:
> yeah I think it’s better than not, but I’d have to try it 😃 
> 
> cautious yes, revert if it’s annoying 😉

See merge request !3124