Add warning message for PocketImport

Warn user if pocket_consumer_key isn't defined

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

@@ -5,6 +5,12 @@
 <div class="row">
     <div class="col s12">
         <div class="card-panel settings">
+            {% if not has_consumer_key %}
+                <div class="card-panel red darken-1">
+                    {% trans %}Pocket import isn't configured. You need to define pocket_consumer_key.{% endtrans %}
+                </div>
+            {% endif %}
+
             <blockquote>{{ import.description|trans }}</blockquote>
             <p>{% trans %}You can import your data from your Pocket account. You just have to click on the below button and authorize the application to connect to getpocket.com.{% endtrans %}</p>
             <form method="post" action="{{ path('import_pocket_auth') }}">