This completely rewrites the SnippetsFinder class from the ground up in
order to improve its performance. The old code was beyond salvaging. It
was complex, included various Rails 5 workarounds, comments that
shouldn't be necessary, and most important of all: it produced a really
poorly performing database query.

As a result, I opted for rewriting the finder from scratch, instead of
trying to patch the existing code. Instead of trying to reuse as many
existing methods as possible, I opted for defining new methods
specifically meant for the SnippetsFinder. This requires some extra code
here and there, but allows us to have much more control over the
resulting SQL queries. It is these changes that then allow us to produce
a _much_ more efficient query.

To illustrate how bad the old query was, we will use my own snippets as
an example. Currently I have 52 snippets, most of which are global ones.
To retrieve these, you would run the following Ruby code:

    user = User.find_by(username: 'yorickpeterse')

    SnippetsFinder.new(user, author: user).execute

On GitLab.com the resulting query will take between 10 and 15 seconds to
run, producing the query plan found at
https://explain.depesz.com/s/Y5IX. Apart from the long execution time,
the total number of buffers (the sum of all shared hits) is around 185
GB, though the real number is probably (hopefully) much lower as I doubt
simply summing these numbers produces the true total number of buffers
used.

The new query's plan can be found at https://explain.depesz.com/s/wHdN,
and this query takes between 10 and 100-ish milliseconds to run. The
total number of buffers used is only about 30 MB.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/52639

This adds a database migration that ensures that
project_features.snippets_access_level defaults to a value of 20 (=
ProjectFeature::ENABLED), instead of NULL. This allows us to simplify
some of the queries used for obtaining snippets, as we no longer need to
handle cases where this column is NULL.

Fixes automatic URL formatting on Maven Page

See merge request gitlab-org/gitlab-ce!22727

Improve markdown and fix links that don't render correctly

See merge request gitlab-org/gitlab-ce!22735

Improve markdown and fix links that don't render correctly

See merge request gitlab-org/gitlab-ce!22737

Improve markdown and fix links that don't render correctly

See merge request gitlab-org/gitlab-ce!22738

Resolve "Filter by `None`/`Any` for milestone in issues/mrs API"

Closes #52382

See merge request gitlab-org/gitlab-ce!22637

Refactor SSH keys docs for Windows clients

Closes #46398 and #46399

See merge request gitlab-org/gitlab-ce!21004

Fix typos in comments and specs

See merge request gitlab-org/gitlab-ce!22683

Reduce SQL queries needed to load open merge requests

See merge request gitlab-org/gitlab-ce!22709

Drop gcp_clusters table

See merge request gitlab-org/gitlab-ce!22713

Whitelist none method from ActiveRecord::Querying

See merge request gitlab-org/gitlab-ce!22726

Prettify remaining files with differences in CE and EE

See merge request gitlab-org/gitlab-ce!22702

Rename @gitlab-org/gitlab-svgs to @gitlab/svgs

See merge request gitlab-org/gitlab-ce!22714

Fix quick links button styles

Closes #52220 and #52063

See merge request gitlab-org/gitlab-ce!22657

Prepare Banzai to work with group issuables

See merge request gitlab-org/gitlab-ce!22711

Build an assets only container

See merge request gitlab-org/gitlab-ce!22235