Improve TBT by rendering content over bigger time span

Changelog: performance

No user facing changes here, only Rubocop
fixes.

Images that were not embedded into Jupyter Notebooks were not being
properly rendered because the path would point to the blob url, not the
raw URL.

Changelog: changed
Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/28601
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/69075

Changelog: added

This is required for the repository viewer to GraphQL work

Changelog: added

Until snippet repo replication is released, this will maintain existing
behavior on Geo secondaries.

Removing the FF snippet_multiple_files and releasing
snippet with multiples files.

Remove the countermeasure that prevented returning
the content.

Also replace as much /snippets occurences with
/-/snippets as possible

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

When the `snippet_multiple_file` feature flag is
enabled, the blob presenter should return paths for
the new raw endpoint (using blob ref and path)

The plan for RSpec 4.0 is to disable monkey patching, reference:
https://rspec.info/blog/2013/07/the-plan-for-rspec-3/#zero-monkey-patching-mode
https://relishapp.com/rspec/rspec-core/docs/configuration/zero-monkey-patching-mode

This commit stops using RSpec monkey patching in several specs.

- Improve variable name
- Add Changelog
- Remove unnecessary let_it_be

If the feature flag is enabled, the snippet has
a repository attached and it's not empty, then we'll use
the blob stored in the repository. If not, we show the snippet
stored in the database.

This behavior will change once we perform the migration.

This commit add the fields blob and blob_viewer to
the graphql snippet type. It is needed by the FE
to use Vue in snippets views.