This style change enforces `return if ...` instead of
`return nil if ...` to save maintainers a few minor review points

These specs were flaky and only passing after a retry due to how
rspec-retry works.

1. The test with paths that don't exist was returning 200 on the first
  try, then 404 on the second, not because the paths don't exist, but
  because the MR IID didn't change, thus the MR couldn't be found.
  I decided to remove the test entirely since we don't seem to return
  404 for paths that don't exist.
2. The test with a user that cannot view the merge request was failing
  the first time because the project owner wasn't removed with
  `project.team.truncate`.
  Changing the `let(:user)` to `create(:user)` and calling
  `project.add_maintainer(user)` in the `before` block fix the test.
Signed-off-by: NRémy Coutable <remy@rymai.me>

This reverts commit a82a5957, reversing
changes made to e7df959b.

**Notes:**
- Also exposes commit.signature_html in diffs.json

Enables frozen string for some vestigial files as
well as the following:

* app/controllers/projects/**/*.rb
* app/controllers/sherlock/**/*.rb
* app/controllers/snippets/**/*.rb
* app/controllers/users/**/*.rb

Partially addresses #47424.

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

add a spec for commit merge request diff notes

The st_commits and st_diffs columns on merge_request_diffs historically held the
YAML-serialised data for a merge request diff, in a variety of formats.

Since 9.5, these have been migrated in the background to two new tables:
merge_request_diff_commits and merge_request_diff_files. That has the advantage
that we can actually query the data (for instance, to find out how many commits
we've stored), and that it can't be in a variety of formats, but must match the
new schema.

This is the final step of that journey, where we drop those columns and remove
all references to them. This is a breaking change to the importer, because we
can no longer import diffs created in the old format, and we cannot guarantee
the export will be in the new format unless it was generated after this commit.

After installing a new gem, batch-loader, a construct can be used to
queue data to be fetched in bulk. The gem was also introduced in both
gitlab-org/gitlab-ce!14680 and gitlab-org/gitlab-ce!14846, but those mrs
are not merged yet.

For the generation of diffs, both the old blob and the new blob need to
be loaded. This for every file in the diff, too. Now we collect all
these so we do 1 fetch. Three `.allow_n_plus_1_calls` have been removed,
which I expect to be valid, but this needs to be confirmed by a full CI
run.

Possibly closes:
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37445
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37599
- https://gitlab.com/gitlab-org/gitlab-ce/issues/37431