Improve some post/discussion permission logic

- Allow users to see their own posts, even if they have been hidden by
someone else
- Don't require hiding a post to be necessarily attributed to a user
- Hide discussions with zero posts, unless the user can edit posts, or
they are the discussion author

src/Core/Discussions/DiscussionsServiceProvider.php

@@ -13,7 +13,9 @@ namespace Flarum\Core\Discussions;
 use Flarum\Core\Search\GambitManager;
 use Flarum\Core\Users\User;
 use Flarum\Events\ModelAllow;
+use Flarum\Events\ScopeModelVisibility;
 use Flarum\Events\RegisterDiscussionGambits;
+use Flarum\Events\ScopeEmptyDiscussionVisibility;
 use Flarum\Support\ServiceProvider;
 use Flarum\Extend;
 use Illuminate\Contracts\Container\Container;
@@ -53,6 +55,19 @@ class DiscussionsServiceProvider extends ServiceProvider
                 }
             }
         });
+
+        $events->listen(ScopeModelVisibility::class, function (ScopeModelVisibility $event) {
+            if ($event->model instanceof Discussion) {
+                if (! $event->actor->hasPermission('discussion.editPosts')) {
+                    $event->query->where(function ($query) use ($event) {
+                        $query->where('comments_count', '>', '0')
+                            ->orWhere('start_user_id', $event->actor->id);
+
+                        event(new ScopeEmptyDiscussionVisibility($query, $event->actor));
+                    });
+                }
+            }
+        });
     }
 
     /**