Since the frontend for this feature isn't ready, better to hide the
confusing field behind a feature flag.

The status is shown for
- The author of a commit when viewing a commit
- Notes on a commit (regular/diff)
- The user that triggered a pipeline when viewing a pipeline
- The author of a merge request when viewing a merge request
- The author of notes on a merge request (regular/diff)
- The author of an issue when viewing an issue
- The author of notes on an issue
- The author of a snippet when viewing a snippet
- The author of notes on a snippet
- A user's profile page
- The list of members of a group/user

This can be done trough the API for the current user, or on the
profile page.

This model will hold the status of a user, including these fields:

- emoji: always present, with a default value
- user: always present, foreign key to user
- message: optional, maximum length of 100

The table also stores

- cached_markdown_version
- message_html

For rendering markdown in the `message` field.

Enable rubocop for db/**/* and ee/db/**/*

Closes #49582

See merge request gitlab-org/gitlab-ce!20834

Back port ReactiveCaching

See merge request gitlab-org/gitlab-ce!20836

doc: Configuring and initializing Helm Tiller

Closes #49025

See merge request gitlab-org/gitlab-ce!20515

Prefer using direct links in docs styleguide

See merge request gitlab-org/gitlab-ce!20832

Adds rack attack disabled by default notice to documentation

See merge request gitlab-org/gitlab-ce!20833

Disable Rack Attack by default

Closes #40973

See merge request gitlab-org/gitlab-ce!16669

Enable frozen string in presenters and policies

See merge request gitlab-org/gitlab-ce!20819

i18n: externalize strings from 'app/views/layouts'

See merge request gitlab-org/gitlab-ce!19766

Bump html-pipeline to 2.8.4

See merge request gitlab-org/gitlab-ce!20828

Replace snake case in SCSS variables

See merge request gitlab-org/gitlab-ce!20799

Use limit parameter to retrieve Wikis from Gitaly

See merge request gitlab-org/gitlab-ce!20764

Fix missing ` in users.md

See merge request gitlab-org/gitlab-ce!20817

Replace author_link snake case in stylesheets, specs, and helpers

Closes #30921

See merge request gitlab-org/gitlab-ce!20797

Add frozen_string_literal to migration templates

See merge request gitlab-org/gitlab-ce!20829

Update doc/ci/docker/using_docker_build.md

Closes #48691

See merge request gitlab-org/gitlab-ce!20426

Part of #47424

In 2.8.3, it was possible to modify a constant without knowing it, which
led to a major performance regression (https://gitlab.com/gitlab-org/gitlab-ce/issues/49409).
Now attempting to modify the HTML::Pipeline::SanitizationFilter.WHITELIST will result in
"can't modify frozen Hash".

See https://github.com/jch/html-pipeline/pull/299.

Add example output for file integrity check rake task

Closes #49244

See merge request gitlab-org/gitlab-ce!20825

Fix: redirect non-SSL request for gitlab registry to valid URL

See merge request gitlab-org/gitlab-ce!20824

HTTP host and URI should not be separated by a colon. Generated URLs was looking like http://registry.examples.com:/foo. That is throwing up let's encrypt cert requests for gitlab registry.

Without this parameter, every load of a Wiki page will load all the Wiki pages
in the repository for the sidebar. This is a significant performance penalty
that can significant slow the display of all Wiki pages.

Relates to #40101

Fix slow Markdown rendering

Closes #49409

See merge request gitlab-org/gitlab-ce!20820

Enable frozen string in:

* app/presenters
* app/policies

Partially addresses #47424.