Consolidate Post visibility logic into the PostPolicy

A post can only be seen if the discussion in which it resides can be
seen. The logic for this belongs in the policy, not the model.

src/Post/Post.php

@@ -12,6 +12,7 @@
 namespace Flarum\Post;
 
 use Flarum\Database\AbstractModel;
+use Flarum\Database\ScopeVisibilityTrait;
 use Flarum\Discussion\Discussion;
 use Flarum\Event\GetModelIsPrivate;
 use Flarum\Event\ScopeModelVisibility;
@@ -42,6 +43,7 @@ use Illuminate\Database\Eloquent\Builder;
 class Post extends AbstractModel
 {
     use EventGeneratorTrait;
+    use ScopeVisibilityTrait;
 
     protected $table = 'posts';
 
@@ -109,28 +111,6 @@ class Post extends AbstractModel
         static::addGlobalScope(new RegisteredTypesScope);
     }
 
-    /**
-     * @param Builder $query
-     * @param User $actor
-     */
-    public function scopeWhereVisibleTo(Builder $query, User $actor)
-    {
-        static::$dispatcher->dispatch(
-            new ScopeModelVisibility($query, $actor, 'view')
-        );
-
-        // Make sure the post's discussion is visible as well
-        $query->whereExists(function ($query) use ($actor) {
-            $query->selectRaw('1')
-                ->from('discussions')
-                ->whereColumn('discussions.id', 'posts.discussion_id');
-
-            static::$dispatcher->dispatch(
-                new ScopeModelVisibility(Discussion::query()->setQuery($query), $actor, 'view')
-            );
-        });
-    }
-
     /**
      * Determine whether or not this post is visible to the given user.
      *

src/Post/PostPolicy.php

@@ -66,6 +66,17 @@ class PostPolicy extends AbstractPolicy
      */
     public function find(User $actor, $query)
     {
+        // Make sure the post's discussion is visible as well.
+        $query->whereExists(function ($query) use ($actor) {
+            $query->selectRaw('1')
+                ->from('discussions')
+                ->whereColumn('discussions.id', 'posts.discussion_id');
+
+            $this->events->dispatch(
+                new ScopeModelVisibility(Discussion::query()->setQuery($query), $actor, 'view')
+            );
+        });
+
         // Hide private posts by default.
         $query->where(function ($query) use ($actor) {
             $query->where('posts.is_private', false)