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

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

Re-add "Update configuration files" section in the 8.4 to 8.5 upgrade guide

Needed because of !2782.

See merge request !2796

Fix milestone order in project and issue edit lists to due_date

_Originally opened at !2579 by @olirogers._

- - -

This MR orders the milestones by due date (earliest first) in the project and edit issue view. These suggestions were made be @orih in #6001 and fixes #6001.

This continues the work of !1934 to order milestones.

I found this annoying when creating milestones, found #6001 but no attached MR. This is my first for this project - comments and criticisms welcome.

See merge request !2780

Needed because of !2782.

[ci skip]

Fix typo in doc/api/builds.md



See merge request !2795

[ci skip]

Validate email addresses using Devise.email_regexp

Also:
- Get rid of legacy `:strict_mode`
- Get rid of custom `:email` validator
- Add some shared examples to spec emails validation

This supersedes !2754 and fixes #3851.

See merge request !2771

API: Allow to set or update a merge-request's milestone

_Originally opened at !2107 by @ skakirill._

- - -


See merge request !2755

Don't show fork button if user can't fork the project.

Fixes #13369 

See merge request !2793

Load all blob data when highlighting content for git-blame.

Fixes #13365

See merge request !2792

Closes #13218

Add a new image in the markdown help page

Previously we were linking directly from the public/
directory. While this worked for the GitLab help page,
it was not working for doc.gitlab.com. Adding an image
in a relative directory and linking from there serves
both ends.

Fixes #13233

See merge request !2790

Previously we were linking directly from the public/
directory. While this worked for the GitLab help page,
it was not working for doc.gitlab.com. Adding an image
in a relative directory and linking from there serves
both ends.

[ci skip]

Remove sqlite check in raketask

This is a relic from 4 major releases ago. Time to remove it.

See merge request !2777

Fixed "Docker Fundamentals" link in GitLab Documentation.

I noticed that the *Docker Fundamentals* link in the *Using Docker Images* part of the GitLab Documentation is broken.

Screenshot:
![broken-link](/uploads/8d2fc782b2f5d4f3316a631f801cd3c7/broken-link.png)

Changing it to "https://docs.docker.com/" should fix the problem.

See merge request !2786

Add merge_requests/:merge_request_id/closes_issues

_Originally opened at !2660 by @Schniz._

- - -

Added `/api/v3/projects/:id/merge_requests/:merge_request_id/closes_issues` route in the API for getting the issues that should be closed when a merge request merged successfully

See merge request !2779

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

Use caching

This enables to use built-in caching feature instead of using the `/cache` folder.

See merge request !2747