Convert english translation file

- convert english translation to translate key
- remove baggy template for login (never used since user isn't logged in and it'll use the default theme: material)
- fix tests about text in response (now checking translation key instead of translated text)
- remove all ugly `<div class="hidden">{{ form_rest(form) }}</div>`

src/Wallabag/ImportBundle/Import/WallabagV2Import.php

@@ -27,7 +27,7 @@ class WallabagV2Import extends WallabagV1Import implements ImportInterface
      */
     public function getDescription()
     {
-        return 'This importer will import all your wallabag v2 articles. Go to All articles, then, on the export sidebar, click on "JSON". You will have a "All articles.json" file.';
+        return 'import.wallabag_v2.description';
     }
 
     /**