Create tables for mentioned users in Markdown

Quoting from #14972 (comment 214739257):

For this to work we need some kind of note_username_mentions table which for every note tracks which usernames it mentions. The structure would be something like this:

• note_id

• user_id (the ID of the mentioned user)

Every time you save a note you'd have to update this table accordingly.

We may also need similar tables for descriptions, as well as notes. When we add this table, we should make sure that it is populated correctly, but not actually read from it otherwise - that will be a in a separate iteration.

Per #21800 (comment 215139619), as part of this issue's scope, it will be populated for every new mentions in issues and mrs and epics.