Refactor translation and validation

We now use Symfony's Translation component. Yay! We get more powerful pluralisation and better a fallback mechanism. Will want to implement the caching mechanism at some point too. The API is replicated in JavaScript, which could definitely use some testing.

Validators have been refactored so that they are decoupled from models completely (i.e. they simply validate arrays of user input). Language packs should include Laravel's validation messages.

ref #267

src/Core/Command/PostReplyHandler.php

@@ -11,6 +11,7 @@
 namespace Flarum\Core\Command;
 
 use Flarum\Core\Access\AssertPermissionTrait;
+use Flarum\Core\Validator\PostValidator;
 use Flarum\Event\PostWillBeSaved;
 use Flarum\Core\Repository\DiscussionRepository;
 use Flarum\Core\Post\CommentPost;
@@ -33,19 +34,27 @@ class PostReplyHandler
      */
     protected $notifications;
 
+    /**
+     * @var PostValidator
+     */
+    protected $validator;
+
     /**
      * @param Dispatcher $events
      * @param DiscussionRepository $discussions
      * @param NotificationSyncer $notifications
+     * @param PostValidator $validator
      */
     public function __construct(
         Dispatcher $events,
         DiscussionRepository $discussions,
-        NotificationSyncer $notifications
+        NotificationSyncer $notifications,
+        PostValidator $validator
     ) {
         $this->events = $events;
         $this->discussions = $discussions;
         $this->notifications = $notifications;
+        $this->validator = $validator;
     }
 
     /**
@@ -79,6 +88,8 @@ class PostReplyHandler
             new PostWillBeSaved($post, $actor, $command->data)
         );
 
+        $this->validator->assertValid($post->getAttributes());
+
         $post->save();
 
         $this->notifications->onePerUser(function () use ($post, $actor) {