Make discussions API work with quick actions-only notes

What does this MR do and why?

This is a follow-up to !173055 (merged).

Previously quick actions were not handled properly in the commits and discussions API for the following API routes:

• POST /projects/:id/<noteable>/<noteable_id>/discussions/:discussion_id/notes
• PUT /projects/:id/issues/:issue_iid/discussions/:discussion_id/notes/:note_id

For example:

• POST /projects/:id/issues/:issue_iid/discussions/:discussion_id/notes
• PUT /projects/:id/issues/:issue_iid/discussions/:discussion_id/notes/:note_id
• POST /projects/:id/repository/commits/:commit_id/discussions/:discussion_id/notes
• PUT /projects/:id/repository/commits/:commit_id/discussions/:discussion_id/notes/:note_id

Users would see a 400 Bad Request even if the quick action were initiated. This commit changes that to respond with a 202 upon success.

References

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Relates to #505100 (closed)

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Before	After

How to set up and validate locally

See the steps in !173055 (merged).