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/Models/Entity.php

@@ -10,6 +10,7 @@ use BookStack\Activity\Models\Loggable;
 use BookStack\Activity\Models\Tag;
 use BookStack\Activity\Models\View;
 use BookStack\Activity\Models\Viewable;
+use BookStack\Activity\Models\Watch;
 use BookStack\App\Model;
 use BookStack\App\Sluggable;
 use BookStack\Entities\Tools\SlugGenerator;
@@ -330,6 +331,14 @@ abstract class Entity extends Model implements Sluggable, Favouritable, Viewable
             ->exists();
     }
 
+    /**
+     * Get the related watches for this entity.
+     */
+    public function watches(): MorphMany
+    {
+        return $this->morphMany(Watch::class, 'watchable');
+    }
+
     /**
      * {@inheritdoc}
      */