Migrate existing implicit mentions to corresponding DB tables
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https://gitlab.com/gitlab-org/gitlab-ce/issues/45125 describes adding a new table (or set of tables) to track mentions of users by note.
Once we've added that table, we need to create a background migration to fill in the data for all existing notes, which will take a very long time, and may need to span a couple of releases.
Also we would need to migrate any mentions in description and title of issues, merge_requests, epics and snippets
Breaking down this issue into smaller issues for each separate migration. Some of the bellow issues still might need to be broken down into several migrations depending of the actual number of records to be migrated.