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

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

Use caching

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

See merge request !2747

Fix CI build status badge

Closes #5570, #4935, #4163

See merge request !2704