Make all Bug objects require a tracker_id

This way, it is technically impossible to create a Bug object that does
not know how to update itself.
In a migration, actively remove all bug objects without a tracker

This is a harsh step to take, but it is required for life in the beautiful future
where every Bug object has a tracker.
Add a migration to force tracker_id to not NULL
Merge branch 'bugs_always_have_trackers'