Whenever you push to a branch GitLab will show a button to create a
merge request (should one not exist already). The underlying code to
display this data was quite inefficient. For example, it involved
multiple slow queries just to figure out what the most recent push event
was.

This commit changes the way this data is retrieved so it's much faster.
This is achieved by caching the ID of the last push event on every push,
which is then retrieved when loading certain pages. Database queries are
only executed if necessary and the cached data is removed automatically
once a merge request has been created, or 2 hours after being stored.

A trade-off of this approach is that we _only_ track the last event.
Previously if you were to push to branch A and B then create a merge
request for branch B we'd still show the widget for branch A. As of this
commit this is no longer the case, instead we will only show the widget
for the branch you pushed to most recently. Once a merge request exists
the widget is no longer displayed. Alternative solutions are either too
complex and/or too slow, hence the decision was made to settle for this
trade-off.

Performance Impact
------------------

In the best case scenario (= a user didn't push anything for more than 2
hours) we perform a single Redis GET per page. Should there be cached
data we will run a single (and lightweight) SQL query to get the
event data from the database. If a merge request already exists we will
run an additional DEL to remove the cache key.

The difference in response timings can vary a bit per project. On
GitLab.com the 99th percentile of time spent in User#recent_push hovers
between 100 milliseconds and 1 second, while the mean hovers around 50
milliseconds. With the changes in this MR the expected time spent in
User#recent_push is expected to be reduced down to just a few
milliseconds.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/35990

Merge branch '30473-allow-creation-of-subgroups-with-gitlab_default_can_create_group-set-to-false' into 'master'

Make Members with Owner and Master roles always able to create subgroups

Closes #30473

See merge request !14046

Add Auto DevOps docs

Closes #37051

See merge request !14031

Update rails template to include postgres

Closes #37156

See merge request !14120

Merge branch '35558-only-one-garbage-collection-should-be-running-per-project-at-once' into 'master'

Adds exclusive lease to Git garbage collect worker.

Closes #35558

See merge request !14036

Resolve RuboCop violations in scripts/trigger-build-docs

See merge request !14118

Restore droplab item style for keyboard navigation

Closes #37218

See merge request !13921

Improve "Share with group lock" feature for subgroups

Closes #30550

See merge request !13944

Resolve "Make project and features visibility settings less confusing"

Closes #32665

See merge request !14062

Resolve "Banner to enable Auto DevOps at project level"

Closes #37158

See merge request !13991

Update docs for confidential issues

Closes #36352

See merge request !14117

Add JSON logger in `log/api_json.log` for Grape API endpoints

Closes #36189

See merge request !14102

Gitaly feature toggles are on by default in development environments

Closes gitaly#485

See merge request !13802

Fix a wrong `X-Gitlab-Event` header when testing webhooks

Closes #37288

See merge request !14108

Implement the implied CI/CD config for AutoDevOps

Closes #34777

See merge request !13923

Decrease Cyclomatic Complexity threshold to 14

See merge request !13972

Removes default scope from sortable

Closes #23079

See merge request !13558

Change strings for projects dropdown placeholder and error state

Closes #37513

See merge request !14079

Make blob viewer wider for mobile

Closes #37368

See merge request !14011

Add option to change navigation color palette

Closes #35012

See merge request !13619

Make dropdown menus on navigation to use full viewport width

See merge request !14086

Add script and job to trigger a docs build

See merge request !13954

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

Finish migration to the new events setup

Closes #37241

See merge request !13932

Fix alignment for Skip to content action

Closes #37530

See merge request !14101

Update schema.rb from db:migrate and fix invalid index check

See merge request !14107