Fix the "x of y" displayed at the top of Issuables' sidebar

1. We now display the index of the current issuable among all its project's
issuables, of the same type and with the same state.
1. Also, refactored a bit the Issuable helpers into a new `IssuablesHelper`
module.
1. Added acceptance specs for the sidebar counter.

Note: I didn't add a CHANGELOG item since it's a bug fix for an unreleased version.

Fixes #13356.

See merge request !2818

Cleaned up Repository#initialize

The "default_branch" argument is never used and the "project" argument
isn't optional.



See merge request !2832

Eager-load image blob data in diffs

Since gitlab_git 8.0, blob data are lazy-loaded so we have to call
`blob.load_all_data!(repo)` to eager-load them.

Fixes #13458.

/cc @jacobvosmaer @dblessing @rspeicher 

### New image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.18](/uploads/01c2afa2d25f3249b27744154ab2069b/Screen_Shot_2016-02-16_at_11.28.18.png)

#### After
![Screen_Shot_2016-02-16_at_11.30.56](/uploads/5ef72f717f22dfd0c1efa9e6d8ed926f/Screen_Shot_2016-02-16_at_11.30.56.png)

### Replaced image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.20](/uploads/39bb61851608643a86d9d7616139fc6c/Screen_Shot_2016-02-16_at_11.28.20.png)

#### After
![Screen_Shot_2016-02-16_at_11.30.59](/uploads/27e754fe8b57a838ac695bace27618c3/Screen_Shot_2016-02-16_at_11.30.59.png)

### Deleted image in commit view

#### Before
![Screen_Shot_2016-02-16_at_11.28.22](/uploads/3648dcc2c36bc644556080e203a29fca/Screen_Shot_2016-02-16_at_11.28.22.png)

#### After
![Screen_Shot_2016-02-16_at_11.31.01](/uploads/2585915d994bb6e245dd8c8553730bbf/Screen_Shot_2016-02-16_at_11.31.01.png)

### Image diff in merge request diff view

#### Before
![Screen_Shot_2016-02-16_at_11.28.37](/uploads/a7963f7139fc91975ca40568570ce4f0/Screen_Shot_2016-02-16_at_11.28.37.png)

#### After
![Screen_Shot_2016-02-16_at_11.31.04](/uploads/6773dbf3b870f082f0262febc9722d20/Screen_Shot_2016-02-16_at_11.31.04.png)

See merge request !2830

Fix cross reference notes  on forks

Updates `cross_reference_exists?` to match on commit only.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/5849

See merge request !2731

Ensure Commit#show responds 404 instead of 500 when given an invalid ID

Closes #13467

See merge request !2823

Properly render the `errors/git_not_found` page



See merge request !2824

The "default_branch" argument is never used and the "project" argument
isn't optional.

Since gitlab_git 8.0, blob data are lazy-loaded so we have to call
blob.load_all_data!(repo) to eager-load them.

Fixes #13458.

Prior, if the request format was, for example, .zip, we'd get an
`ActionView::MissingTemplate` error.

Closes #13467

Fix undefined method `postgresql?` during migration

See merge request !2820

1. We now display the index of the current issuable among all its project's
issuables, of the same type and with the same state.
2. Also, refactored a bit the Issuable helpers into a new IssuablesHelper
module.
3. Added acceptance specs for the sidebar counter.

Add notice about offline migrations

/cc @sytses 

See merge request !1723

Fix duplicate "me" in tooltip of the "thumbsup" awards Emoji

Steps to reproduce:

1. Go to a MR or issue
2. Click on "thumbsup" Emoji 3 times
3. Observe the tooltip becomes "me, me"

Closes #13374, #12788

See merge request !2804

Update omniauth to 1.3.1 for memory performance

OmniAuth 1.3.1 may improve memory performance (~ 70% according to [intridea/omniauth!774](https://github.com/intridea/omniauth/pull/774)).

Closes #2404.

See merge request !2805

Fix broken link in CI build notification emails

Closes #13199

See merge request !2785

Fix relative links in other markup formats

_Originally opened at !1845 by @ben.boeckel._

- - -

The RelativeLinkFilter was not applied to other document formats, e.g.,
reStructuredText, so links from the Files view or the Project view did
not work.

Fixes #3533.

See merge request !2798

Steps to reproduce:

1. Go to a MR or issue
2. Click on "thumbsup" Emoji 3 times
3. Observe the tooltip becomes "me, me"

Closes #13374, #12788

Remove note on backing up GitLab CI as a separate service

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/13405

See merge request !2803

[ci skip]

Fix duplicated branch creation/deletion Web hooks/service notifications when using Web UI

Similar to 423d2d62, except duplicates occurred only if a Web service (e.g. Slack) were
configured.

When deleting a branch, this is what was happening:

    1. DeleteBranchService calls execute_hooks and execute_services
    2. The call to repository.rm_branch triggers the GitHooksService.
    3. This, in turn, calls GitPushService and then calls the same hooks/services again.

5145706c now makes it no longer necessary for DeleteBranchService and CreateBranchService to execute
the branch hooks/services. Note that tags behave differently in GitTagPushService and GitPushService
is not called.

Closes #10330

See merge request !2763

Don't try to mark broken MR as mergable

In case merge request is broken, we shouldn't check if the sha
is mergable, as it will be null, and there's no point, as we know
that it's not mergable.

See merge request !2783

Correct indentation for `issues/merge_requests` partial render

Prior, it would only be shown if the issue had been edited.

See merge request !2789

- Apply the RelativeLinkFilter filter to other formats, e.g.,
  reStructuredText so links from the Files view or the Project view work
- Remove the AsciidocPipeline pipeline

Fixes #3533.

Loosen Nokogiri version requirement

Closes #13218

See merge request !2791

This also extracts shared examples for notification emails to separate
file.

Closes #13199

Use gitlab-workhorse 0.6.4

This fixes a bug in 8.5.0.rc1 where downloading Git blobs through the API is broken.

See merge request !2794