Adds `# frozen_string_literal: true` to spec/models ruby files

This commit, introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23812,
fixes a problem creating a displaying image diff notes when the image
is stored in LFS. The main problem was that `Gitlab::Diff::File` was
returning an invalid valid in `text?` for this kind of files.

It also fixes a rendering problem with other LFS files, like text
ones. They LFS pointer shouldn't be shown when LFS is enabled
for the project, but they were.

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

Signed-off-by: Rémy Coutable <remy@rymai.me>