- Adds filtering to the watched items list in notification preferences
so that deleted (recycle bin) items are removed via query.
- Adds relations and logic to properly remove watches upon user and
entity delete events, to old watches in database do not linger.
- Adds testing to cover the above.
Did not add migration for existing data, since patch will be close to
introduction, and lingering DB entries don't open a security concern,
just some potential confusion in specific potential scenarios.
Probably not work extra migration risk, although could add in future if
concerns/issues are found.
Related to #4499
