Fix live notes refresh in relative URL deployments

### What does this MR do?

This MR fixes URLs used for refreshing notes--the relative_url prefix was appended twice

### Why was this MR needed?

In case of `relative_url` installation, opening a page which does refresh notes (i.e. merge request), the page generates lots of 404 errors in the browser console. It turns out that the relative_url is prepended to the link twice.

Supersedes !233

See merge request !1048

Fix the "Reload with full diff" URL button

On the first load, the merge request tabs would be requested in JSON format to render
the HTML. This would cause the "Reload with full diff" button to create a link to `diff.json`. Instead, generate the path in HTML mode.

Closes #2041

See merge request !1019

Dynamically check `:admin_*` ability in two more spots.

Closes #2092 

Depends on !1037 

See merge request !1042

Fix error 500 when no HEAD is available

Steps to reproduce:

1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Error:

```
NoMethodError - undefined method `sha' for nil:NilClass:
  app/helpers/projects_helper.rb:281:in `readme_cache_key'
  app/views/projects/_readme.html.haml:10:in `_app_views_projects__readme_html_haml___2036282917939462960_70154565285700'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:339:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:143:in `render'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiling_methods.rb:108:in `block in profile_method'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:306:in `render_partial'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:279:in `block in render'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:278:in `render'
  actionview (4.1.11) lib/action_view/renderer/renderer.rb:47:in `render_partial'
  actionview (4.1.11) lib/action_view/helpers/rendering_helper.rb:35:in `render'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `block in render_with_haml'
  haml (4.0.5) lib/haml/helpers.rb:89:in `non_haml'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `render_with_haml'
  app/views/projects/show.html.haml:47:in `_app_views_projects_show_html_haml__1458303859692972698_70154948164180'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
```

Closes https://github.com/gitlabhq/gitlabhq/issues/9484, but there may be some other issue there.

See merge request !1041

Add admin_merge_request ability to Developer level and up

This was necessary because this permission is checked dynamically by
`app/views/shared/issuable/_context` when on a Merge Request.

Closes #2058

See merge request !1037

Closes #2092

Closes #2092

This was necessary because this permission is checked dynamically by
`app/views/shared/issuable/_context` when on a Merge Request.

Closes #2058

Steps to reproduce:
1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Closes https://github.com/gitlabhq/gitlabhq/issues/9484

Allow custom label to be set for authentication providers.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2341

Beside the above, I've:
- Refactored `OauthHelper` to have clearer method names and behaviour
- Moved some of `OauthHelper` behaviour to `Gitlab::OAuth::Provider`
- Renamed `OauthHelper` to `AuthHelper` since LDAP, SAML, Kerberos aren't OAuth
- Updated the icons for GitHub and GitLab

In the examples below, "OurAuth" is a SAML provider with a custom label.

![Screen_Shot_2015-07-02_at_16.29.52](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7d425bde69dc34e1667ebd5375d0266d/Screen_Shot_2015-07-02_at_16.29.52.png)

![Screen_Shot_2015-07-02_at_16.31.40](https://gitlab.com/gitlab-org/gitlab-ce/uploads/cbb273321ecdf4cab3d3ef0dc35553e7/Screen_Shot_2015-07-02_at_16.31.40.png)

![Screen_Shot_2015-07-02_at_16.32.39](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d8dd6e1d0dc45a788e869cdcdc99e178/Screen_Shot_2015-07-02_at_16.32.39.png)

![Screen_Shot_2015-07-02_at_16.33.18](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7dbfe8b0ae229c32a08d6c7442976d83/Screen_Shot_2015-07-02_at_16.33.18.png)


See merge request !927

Fix and style the "Changes suppressed" link.

At some point the "Changes suppressed. Click to show" link stopped behaving as expected, so this fixes it.

I also thought it looked bad:

![Screen_Shot_2015-07-23_at_4.06.49_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0a1552e4edb659f065bd202b93bc61ef/Screen_Shot_2015-07-23_at_4.06.49_PM.png)

So I gave it some room to breathe:

![Screen_Shot_2015-07-23_at_4.00.25_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/abc0704e597ed010cade81782ac9099e/Screen_Shot_2015-07-23_at_4.00.25_PM.png)


See merge request !1038

Make username parameter for Users#show case-insensitive

Closes #2012

See merge request !996

On the first load, the merge request tabs would be requested in JSON format to render
the HTML. This would cause the "Reload with full diff" button to create a link to diff.json.

Closes #2041

Update changelog in master



See merge request !1036

Changes for gmail inbox actions

Related to the ongoing efforts to get Gmail inbox actions for GitLab.com in #1517

Not using `mdash` would break the rendering of the html email by switching `Content-Transfer-Encoding` to `quoted-printable` for every email.

Removed the rake task and docs to prevent users from sending dummy emails which might cause application rejection.

See merge request !1024

Fix bug causing "Remove source-branch" option not to work for merge requests from the same project.

Fixes #2049.

See merge request !1025

Revert cache for events

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1014#note_1661322

See merge request !1027

Revert cache issue

It reverts https://dev.gitlab.org/gitlab/gitlabhq/commit/40e31e1db31e6ba9a1cd6255afc2f09df61f4060

and fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/2048
and https://gitlab.com/gitlab-org/gitlab-ce/issues/2043

See merge request !1028

Fix link to 2fa help page.  Closes #2055

See merge request !1026

Check if session_expire_delay column exists before adding the column.

Fixes #2052 

Fixes #1985

See merge request !1029

Gitlab Issue 707: Indent unfolded code 1 character

revert caching of note text

This MR reverts the caching of note texts because what is rendered as a link in Markdown depends on the user's access to other projects and groups.

Until we find a solution that doesn't decrease security, let's not use the cache for this.

See discussion in gitlab-org/gitlab-ce!911 for more information.

/cc @DouweM @stanhu @vsizov

See merge request !1014