Closes #42877.

Immediately using #from here requires a lot of changes in
other finders (e.g. IssuableFinder, TodosFinder). In all places where we
use #merge, this goes completely the wrong way when passed in a relation
that was built with `#from(...)`: The original query's FROM part gets
completely replaced.

This avoids changing all other places and focuses on improving
SnippetFinder with the downside of two (small) codepaths to do the same
thing.

This helps with queries that get project ids based on the - comparably
rare - visibility levels 10 and 20. For these, postgres can now leverage
the partial index for a index-only scan to improve performance.

Example queries:
SELECT id FROM projects WHERE visibility_level IN (10,20)
SELECT id FROM projects WHERE visibility_level IN (10)

For MySQL, this results in a full index on id because MySQL omits the
WHERE clause. That is, the index is a duplicate of the primary key
basically.

Move spinach project network graph tests to RSpec

See merge request gitlab-org/gitlab-ce!17171

Remove unecessary validate: true from belongs_to :project

See merge request gitlab-org/gitlab-ce!17166

Remove graphs_show.js webpack bundle

See merge request gitlab-org/gitlab-ce!17194

This does not seem to serve any clear purpose and causes other issues (see https://gitlab.com/gitlab-org/gitlab-ce/issues/43196#note_59275652)

Remove extraneous tests from Issues API spec

Closes #25201

See merge request gitlab-org/gitlab-ce!17222

Update two links in the changelog developer docs

See merge request gitlab-org/gitlab-ce!17220

Chart.html.haml refactor

See merge request gitlab-org/gitlab-ce!16897

These were all testing an implementation detail of
`Issues::UpdateService` which is already well-tested and doesn't need to
be re-tested here.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/25201

[GH Import] Create an empty wiki if wiki import failed

Closes #43020

See merge request gitlab-org/gitlab-ce!17112

Resolve "group request membership mail with too long list of "To:""

Closes #42274

See merge request gitlab-org/gitlab-ce!17121

Fix single digit value clipping for stacked progress bar

Closes gitlab-ee#4864

See merge request gitlab-org/gitlab-ce!17217

Avoid slow File Lock checks when not used

Closes #43339

See merge request gitlab-org/gitlab-ce!17140

Increase feature flag cache TTL to one hour

Closes #40854

See merge request gitlab-org/gitlab-ce!17213

Fix and simplify end-to-end tests for secret variables

Closes gitlab-qa#190

See merge request gitlab-org/gitlab-ce!17173

Flipper already takes care of flushing cache entries when
enabling/disabling features so it should be safe to increase the TTL.
This in turn should drastically reduce the number of Flipper queries
executed.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/40854

Resolve "Double closing system  notes when closing issue with Merge Request"

Closes #32546 and #32564

See merge request gitlab-org/gitlab-ce!17035

Don't attempt to update user tracked fields if database is in read-only

Closes gitlab-ee#4972

See merge request gitlab-org/gitlab-ce!17200

The tracking issue if this causes problems is
https://gitlab.com/gitlab-org/gitaly/issues/1028

Docs: Update all articles with the new layout (metadata from the frontmatter)

Closes #42831

See merge request gitlab-org/gitlab-ce!17187

Fix squash rebase not working when diff contained encoded data

Closes gitlab-ee#4960

See merge request gitlab-org/gitlab-ce!17205