In Runner v1.3.0 we've started to send User-Agent header with Runner's
version data. Since GitLab v8.12.0 we've started to use this header to check
if used Runner's version supports 204 status code instead of 404 as a
response when there is no jobs to execute by a Runner.

In APIv4 (introduced in GitLab 9.0.0) will require Runner v9.0.0. And
writing more accurately: GitLab Runner v9.0.0 will require GitLab at
least 9.0.0. Because of such breaking change we are able to switch
entirely to 204 response code and there is no need to do check of
User-Agent.

This commit removes useless code and complexity.

In APIv1 we were using UpdateJob to verify if the runner exists. It was
the only method that was using Runner's token and used in special way
had no side effects (like scheduling a new job or unregisterring a
Runner).

In APIv4 we've change UpdateJob to use job's token as authentication
credentials, and that way we've removed the only endpoint that could
be used to verify if the Runner with a certain token exists in target
GitLab installation.

This commit adds `POST /api/v4/runners/verify` endpoint whose only
responsibility is to respond if Runner with posted credentials exists or
not.

See merge request !9623.
Signed-off-by: Rémy Coutable <remy@rymai.me>

Rename 'All issues' to 'Open issues' in Add issues modal

Closes #29617

See merge request !10042

don't show loading spinners for server-rendered sidebar fields

Closes #24683

See merge request !9983

All CI offline migrations

See merge request !9730

26236 Change gfm textarea to use monospace font

Closes #26236

See merge request !10021

add changelog entry

Remove width from nav links

Closes #29586

See merge request !10051

Fixes diff comment avatar tooltip escaping issues

Closes #29572

See merge request !10012

Resolve "Extract logic of who should receive notification into separate classes"

Closes #27477

See merge request !9446

Added Gitlab::Database.config

See merge request !10045

Use Gitaly for CommitController#show

See merge request !9629

Resolve GitLab QA cold boot problems on entry page

See merge request !10003

Remove UJS actions from pipelines tables

Closes #20450, #28535, and #5580

See merge request !9929

Fix time-sensitive helper spec

See merge request !10046

Fix Unicode 1.1 emojis

Closes #29557

See merge request !10031

This returns the ActiveRecord configuration for the current environment.

While CE doesn't use this very often, EE will use it in a few places for
the database load balancing code. I'm adding this to CE so we don't end
up with merge conflicts in this file.

Add a new have_html_escaped_body_text that match an HTML-escaped text

Closes #20121

See merge request !10015

Signed-off-by: Rémy Coutable <remy@rymai.me>

Stop CI notification showing when status is nil

See merge request !10044

Updates realtime documentation for the Frontend

See merge request !10038

Refactor award emojis document

Closes #29563

See merge request !10040

Issue boards blank state template in JS file

See merge request !9959

Restore sub-nav (Activity) for empty project

Closes #29577

See merge request !10033

Added labels to the issue web hook

Closes #22313

See merge request !9972

Use "branch_name" instead "branch" on V3 branch creation API

Closes #29604

See merge request !10030

Add ability to disable Merge Request URL on push

Closes #21451

See merge request !9663

Strip reference prefixes on branch creation

Closes #26470

See merge request !8498