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

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: NDmitriy 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

These changes are useful for CE as well. For example, the MR
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20720 can make
use of some of the backported methods.

adds read_repository scope description

See merge request gitlab-org/gitlab-ce!20707

Bump nokogiri to 1.8.4 and sanitize to 4.6.6 for performance

See merge request gitlab-org/gitlab-ce!20795

Enable frozen string in app/serializers/**/*.rb

See merge request gitlab-org/gitlab-ce!20726

Checks that new Ruby files will enable
frozen string literal.

Partially addresses #47424.

Add docs to disable proxy listener

See merge request gitlab-org/gitlab-ce!19734

Revert "Merge branch '2211-add-option-to-change-clone-path-docs' into 'master'"

See merge request gitlab-org/gitlab-ce!20633

In docs, add `No+Milestone` for querying by milestone

See merge request gitlab-org/gitlab-ce!20784