Add warning message for import + download images

When import isn't async and downloading images is enabled, we warn the user that import can fail.
Also, added missing translations

src/Wallabag/ImportBundle/Resources/views/Firefox/index.html.twig

@@ -6,6 +6,8 @@
 <div class="row">
     <div class="col s12">
         <div class="card-panel settings">
+            {% include 'WallabagImportBundle:Import:_information.html.twig' %}
+
             <div class="row">
                 <blockquote>{{ import.description|trans|raw }}</blockquote>
                 <p>{{ 'import.firefox.how_to'|trans }}</p>