Make MappedMorphTo available on all models

In case extensions want to add that kind of relationship to an existing
model (there’s no way to include traits at runtime)

src/Core/Models/Model.php

@@ -5,10 +5,12 @@ use Illuminate\Database\Eloquent\Model as Eloquent;
 use Flarum\Core\Exceptions\ValidationFailureException;
 use Flarum\Core\Exceptions\PermissionDeniedException;
 use Flarum\Core\Support\EventGenerator;
+use Flarum\Core\Support\MappedMorphToTrait;
 
 class Model extends Eloquent
 {
     use EventGenerator;
+    use MappedMorphToTrait;
 
     /**
      * Disable timestamps.

src/Core/Models/Notification.php

@@ -1,11 +1,7 @@
 <?php namespace Flarum\Core\Models;
 
-use Flarum\Core\Support\MappedMorphToTrait;
-
 class Notification extends Model
 {
-    use MappedMorphToTrait;
-
     /**
      * The table associated with the model.
      *