Show merge errors in merge request widget

Closes #28124 and gitlab-ee#1652

See merge request !9229

Set `Auto-Submitted: auto-generated` header to emails

Closes #20305

See merge request !9163

There were two problems here:

1. On the JS side, the reference to $widgetBody didn't refer to the
   right DOM element any more. This might be because it was replaced by
   the `getMergeStatus` method. Even if it wasn't, ensuring we have the
   right element means that the content gets updated.

2. On the Ruby side, the `log_merge_error` method didn't update the
   `merge_error` column of the merge request. Change that to update if
   requested, and update in the most common cases by default.

   Additionally, this would sometimes return an error hash, but it
   doesn't look like this was ever used (the return value of
   `MergeService#execute` appears to be unused everywhere).

Clarify that stage is needed to stop environments

Closes #28110

See merge request !9296

[ci skip]

Fix broken link in university docs

See merge request !9295

[ci skip]

Make Karma output look nicer for CI

See merge request !9165

Clean up terms used for issues. Use New instead of Add.

See merge request !9281

style links on system notes to look clickable

Closes #27959

See merge request !9251

Fix most of broken docs links

See merge request !9275

Change wording of MR comment box

See merge request !9193

[ci skip]

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/20305

Move tag services to `Tags` namespace

See merge request !9231

Fix Rubocop offense

See merge request !9274

Fix regression where cmd-click stopped working for todos and merge request tabs

Closes #27922

See merge request !9115

Resolve 'Alphabetically sort tags on runner list'

Closes #27409

See merge request !8922

Remove deprecated CI service

Closes #28094

See merge request !9221

Specify that only project owners can transfer a project

Closes #26651

See merge request !9262

Add missing api entities

Closes #23895

See merge request !7138

Only load istanbul plugin in development mode

See merge request !9261

Stop Pikaday using moment

See merge request !9244

Show notifications settings even when repository feature is disabled

See merge request !9180

# Conflicts:
#   db/schema.rb

Fix labels being applied to wrong merge requests on GitHub import

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2064

Fix for XSS vulnerability in SVG attachments

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2059

Fix XSS in rdoc and other markups

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2058

Add sanitization filter to asciidocs output to prevent XSS

See https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/2057

Centers loading icon vertically and horizontally in pipelines table in commit view

Closes #28229

See merge request !9256

[skip ci]