Backports relevant changes made in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5163 to CE

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

The sanitize transformers were being duplicated each time the Markdown
renderer was called, leading to expontential growth in rendering times.

The problem was that although HTML::Pipeline::SanitizationFilter.WHITELIST
is a frozen hash, the `:transformers` array can be modified. We need
to do deep copy of this to avoid adding duplicates.

Closes #49409

Add best practice points regarding background migrations on huge tables

See merge request gitlab-org/gitlab-ce!20815

Fix typo in high availability docs

See merge request gitlab-org/gitlab-ce!20805

Remove extra parenthesis in PAT docs

See merge request gitlab-org/gitlab-ce!20807

Backport various EE changes to Gitlab::Database

See merge request gitlab-org/gitlab-ce!20809

Add :sidekiq group to cleanup redis in after callback

See merge request gitlab-org/gitlab-ce!20816

This is already applied in EE

Create class responsible for default object store settings

See merge request gitlab-org/gitlab-ce!20785

Remove timeouts for long running requests

See merge request gitlab-org/gitlab-ce!20810

Check Frozen String Literal in Danger

See merge request gitlab-org/gitlab-ce!20767

Add an option to have a private profile on GitLab

See merge request gitlab-org/gitlab-ce!20387

Make repository cleanup mandatory

Closes #49550

See merge request gitlab-org/gitlab-ce!20808

Update improve issue boards links to docs [ci skip]

See merge request gitlab-org/gitlab-ce!20803

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fix Vue datatype errors for markdownVersion parsing

See merge request gitlab-org/gitlab-ce!20800

Show decimal place up to single digit in Stacked Progress Bar

Closes gitlab-ee#6028

See merge request gitlab-org/gitlab-ce!20776