warning bar: add parameter to dis/enable it

src/Wallabag/CoreBundle/Resources/views/base.html.twig

@@ -113,6 +113,10 @@
                 <p>{% trans %}powered by{% endtrans %} <a href="http://wallabag.org">wallabag</a></p>
             </footer>
         {% endblock %}
-        <div id="alphabar">You're trying wallabag v2, which is in alpha version. If you find a bug, please have a look to <a href="https://github.com/wallabag/wallabag/issues">our issues list</a> and <a href="https://github.com/wallabag/wallabag/issues/new">open a new if necessary</a>.</div>
+        {% if warning_message %}
+        <div id="warning_message">
+            {{ warning_message | raw }}
+        </div>
+        {% endif %}
     </body>
 </html>

src/Wallabag/CoreBundle/Resources/views/themes/material/public/css/main.css

@@ -26,7 +26,7 @@ body {
     background: #f0f0f0;
 }
 
-#alphabar {
+#warning_message {
     position: fixed;
     background-color: #ff6347;
     z-index: 1000;
@@ -36,7 +36,7 @@ body {
     color: #000;
 }
 
-#alphabar a {
+#warning_message a {
     color: #555;
 }