Eliminate many Gitaly calls in discussions API

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

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