I didn't think this change through and it's going to be too difficult
to implement right now. It can wait until we do the notifications
revamp. For now reverting back to the old structure, with the
`sender_id` column renamed to `from_user_id`.
* Make filenames and order more consistent
* Split foreign keys into their own migrations, add statements to ensure
data integrity prior to adding them
* Add renameColumns helper, use other helpers where possible
