Improve ProcessCommitWorker for large push payloads

Closes #25827

See merge request !8267

Fix errorcode 500 on slash commands

Closes #26013

See merge request !8285

[ci skip] Fix wrong release date for 8.15.0 on CHANGELOG

See merge request !8294

When processing push payloads we now schedule at most the 100 most
recent commits, instead of all commits that were in a payload. This
prevents one from overloading the system by pushing thousands if not
millions of commits in a single go.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25827

Fix code that selects latest pipelines

Closes #25993 and #26031

See merge request !8286

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8286/diffs#note_20435082

Post username as property, not user_name

See merge request !8261

Feedback from Grzegorz

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8286/diffs#note_20433402

Fix state_event parameter to reopen an issue

Closes #25958

See merge request !8246

The name latest implies that it's reverse chronological,
and we did expect it that way.

https://gitlab.com/gitlab-org/gitlab-ce/issues/25993#note_20429761


> ok, I think markglenfletchera is correct in
> https://gitlab.com/gitlab-com/support-forum/issues/1394#note_20399939
> that `Project#latest_successful_builds_for` is giving oldest pipeline
> rather than latest pipeline. This is a ~regression introduced by !7333
> where `order(id: :desc)` was removed causing this. The offending change
> was:
> https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7333/diffs#b22732e5f39e176c7c719fe485847d0fb0564275_92_108
>
> The confusion was caused by the `latest` name implication, which
> actually didn't order anything, and I think we should add `order(id:
> :desc)` to `Ci::Pipeline.latest` otherwise it's confusing that it's not
> actually ordered.


Closes #25993

Coverage was failing, so this could slip through the cracks

Add more images to issue creation from unresolved discussions

See merge request !8279

Fixed typo in test-and-deploy-ruby-application-to-heroku.md

See merge request !8236

[ci skip]

Change earlier to task_status_short to avoid titlebar line wraps

Closes #25941

See merge request !8275

Stops GFM special characters interfering with markdown tags

See merge request !8265

Add Gitaly to the architecture of the application

See merge request !8264

Slack docs [ci skip]

See merge request !8269

Resolve "Labels are not consistent on all pages"

Closes #25951

See merge request !8256

[ci skip]

Increase left padding of filter row labels

See merge request !8258

[ci skip]

Fix viewing "build failed" TODOs

Closes #25979

See merge request !8262

Docs on MM auto config

See merge request !8230

Fix format of Slack when result is nil

Closes #25954

See merge request !8248

Fix hover pipeline

See merge request !8253

Fix dropdown content non appearing in MR view

Closes #25961

See merge request !8255

Improve autodeploy docs

See merge request !8242

Fix Mattermost command creation by specifying username

See merge request !8257

Added both to the graph, and to the high level overview. Explaining what
is the goal of the system and how does it connect to the other systems.

Don't render inline math when dollar signs are inside markup

See merge request !8259