Bring back "Migrate epic mentions to epic_user_mentions"

This brings back !22333 (merged), which was reverted in !23900 (merged) due to a broken master (#199297 (closed)).

This also fixes the migration in migrate_epic_notes_mentions_to_db_spec.rb. The spec for the migration assumed that the primary ID for notes started at 1, but if another migration ran before it that would change the sequence. We can fix this by hard-coding the IDs in the spec to start at 1.

Closes #199297 (closed)