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

tests/Helpers/EntityProvider.php

@@ -11,6 +11,7 @@ use BookStack\Entities\Repos\BookRepo;
 use BookStack\Entities\Repos\BookshelfRepo;
 use BookStack\Entities\Repos\ChapterRepo;
 use BookStack\Entities\Repos\PageRepo;
+use BookStack\Entities\Tools\TrashCan;
 use BookStack\Users\Models\User;
 use Illuminate\Database\Eloquent\Builder;
 
@@ -197,6 +198,16 @@ class EntityProvider
         return $draftPage;
     }
 
+    /**
+     * Fully destroy the given entity from the system, bypassing the recycle bin
+     * stage. Still runs through main app deletion logic.
+     */
+    public function destroy(Entity $entity)
+    {
+        $trash = app()->make(TrashCan::class);
+        $trash->destroyEntity($entity);
+    }
+
     /**
      * @param Entity|Entity[] $entities
      */