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/Http/Controller/ClientView.php

@@ -96,22 +96,22 @@ class ClientView implements Renderable
     protected $request;
 
     /**
-     * @var \Flarum\Asset\AssetManager
+     * @var AssetManager
      */
     protected $assets;
 
     /**
      * @var JsCompiler
      */
-    protected $locale;
+    protected $localeJs;
 
     /**
      * @param Client $api
      * @param Request $request
      * @param User $actor
-     * @param \Flarum\Asset\AssetManager $assets
+     * @param AssetManager $assets
      * @param string $layout
-     * @param JsCompiler $locale
+     * @param JsCompiler $localeJs
      */
     public function __construct(
         Client $api,
@@ -119,14 +119,14 @@ class ClientView implements Renderable
         User $actor,
         AssetManager $assets,
         $layout,
-        JsCompiler $locale = null
+        JsCompiler $localeJs = null
     ) {
         $this->api = $api;
         $this->request = $request;
         $this->actor = $actor;
         $this->assets = $assets;
         $this->layout = $layout;
-        $this->locale = $locale;
+        $this->localeJs = $localeJs;
     }
 
     /**
@@ -214,7 +214,7 @@ class ClientView implements Renderable
     /**
      * Get the view's asset manager.
      *
-     * @return \Flarum\Asset\AssetManager
+     * @return AssetManager
      */
     public function getAssets()
     {
@@ -264,8 +264,8 @@ class ClientView implements Renderable
         $view->styles = [$this->assets->getCssFile()];
         $view->scripts = [$this->assets->getJsFile()];
 
-        if ($this->locale) {
-            $view->scripts[] = $this->locale->getFile();
+        if ($this->localeJs) {
+            $view->scripts[] = $this->localeJs->getFile();
         }
 
         $view->head = implode("\n", $this->headStrings);