Watching: Prevent issues when watchable or user is deleted

- 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

app/Entities/Tools/TrashCan.php

@@ -376,6 +376,7 @@ class TrashCan
         $entity->searchTerms()->delete();
         $entity->deletions()->delete();
         $entity->favourites()->delete();
+        $entity->watches()->delete();
         $entity->referencesTo()->delete();
         $entity->referencesFrom()->delete();