fix handling of empty ref_name parameter string in commits api

Closes #64745

See merge request gitlab-org/gitlab-ce!31687

Eliminate many Gitaly calls in discussions API

Closes #65957

See merge request gitlab-org/gitlab-ce!31834

Previously, the API to retrieve discussions from merge requests often
generated hundreds of Gitaly calls to determine whether a system note
should be shown to the user. It did this by:

1. Rendering the Markdown
2. Extracting cross-references from the Markdown
3. For cross-references that were commits, a Gitaly FindCommit RPC
   would be issued to validate that the commit exists.

The last step is unnecessary because we don't need to display a commit
if the user doesn't have access to the project in the first place.

`RendersNotes#prepare_notes_for_rendering` is already used in
`MergeRequestsController`, which is why we don't see N+1 Gitaly calls
there. We use it here to optimize the note redaction process.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65957

Adds EE prepend, which for some reason was missing

See merge request gitlab-org/gitlab-ce!31851

This was added in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31789

CE-specific changes for:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/15129

Co-Authored-By:    Alex Kalderimis <akalderimis@gitlab.com>
Co-Authored-By:    Luke Duncalfe <lduncalfe@eml.cc>

This introduces several changes, but these are all just ported from the
EE project.

when params[:ref_name] is set to "" by passing an empty query parameter
to the api it is evaluated as false by the || operator.
The use of active support core extensions presence method fixes the original
implemantation.

https://guides.rubyonrails.org/active_support_core_extensions.html#presence

Resolve EE differences in the lib/ directory

See merge request gitlab-org/gitlab-ce!31582