Only add posted activity if the post is visible publicly

This is very restrictive behaviour and we'll probably need to think of
something a bit more powerful in the future. But it's a good stopgap.

src/Core/Activity/Listeners/UserActivitySyncer.php

@@ -5,6 +5,7 @@ use Flarum\Core\Activity\PostedBlueprint;
 use Flarum\Core\Activity\StartedDiscussionBlueprint;
 use Flarum\Core\Activity\JoinedBlueprint;
 use Flarum\Core\Posts\Post;
+use Flarum\Core\Users\Guest;
 use Flarum\Events\PostWasPosted;
 use Flarum\Events\PostWasDeleted;
 use Flarum\Events\PostWasHidden;
@@ -95,9 +96,11 @@ class UserActivitySyncer
      */
     protected function postBecameVisible(Post $post)
     {
-        $blueprint = $this->postedBlueprint($post);
+        if ($post->isVisibleTo(new Guest)) {
+            $blueprint = $this->postedBlueprint($post);
 
-        $this->activity->sync($blueprint, [$post->user]);
+            $this->activity->sync($blueprint, [$post->user]);
+        }
     }
 
     /**

src/Core/Posts/Post.php

@@ -3,6 +3,7 @@
 use DomainException;
 use Flarum\Events\PostWasDeleted;
 use Flarum\Core\Model;
+use Flarum\Core\Users\User;
 use Flarum\Core\Support\Locked;
 use Flarum\Core\Support\VisibleScope;
 use Flarum\Core\Support\EventGenerator;
@@ -96,6 +97,25 @@ class Post extends Model
         static::addGlobalScope(new RegisteredTypesScope);
     }
 
+    /**
+     * Determine whether or not this post is visible to the given user.
+     *
+     * @param User $user
+     * @return boolean
+     */
+    public function isVisibleTo(User $user)
+    {
+        $discussion = $this->discussion()->whereVisibleTo($user)->first();
+
+        if ($discussion) {
+            $this->setRelation('discussion', $discussion);
+
+            return (bool) $discussion->postsVisibleTo($user)->find($this->id)->count();
+        }
+
+        return false;
+    }
+
     /**
      * Define the relationship with the post's discussion.
      *