- Use existing `selectRaw()` method to avoid using the global `app()`
helper as a service locator, which hides dependencies.
- Do the same for the join.
- The `Expression` is necessary to prevent the aliased column from being
prefixed with the database table prefix, if configured.
This will prevent a notification from being seen by a user if its
subject is deleted or undergoes some kind of permission change (eg.
a discussion is moved into a private tag)
ref #1380
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`.
