Add update_my_username

[ahmadsyafiqruhazat]

No description provided.

[ruqqq]

By right, the User table should have up to date username because Scriber would do an upsert for every messages in receives. Can you check if that would be sufficient to cover the use case you're thinking about?

In terms of the @mention thanks not working due to username can't be found in db, the only case I think this would happen is if the person being mentioned have not been active in any of msociety rooms AND have changed their username.