Fix flagging debates and comments with AJAX

We weren't using `foundation()` in these cases, so after flagging a
debate or a comment, we had to reload the page before we could unflag
it.

We're also adding a test for the fix in commit ea85059d. This test shows
it's necessary to filter the elements with JavaSctipt using `first()` if
we want the same code to work with comments.

Co-Authored-By: taitus <sebastia.roig@gmail.com>

app/assets/javascripts/flaggable.js

@@ -2,7 +2,7 @@
   "use strict";
   App.Flaggable = {
     update: function(resource_id, button) {
-      $("#" + resource_id + " .js-flag-actions").html(button).foundation();
+      $("#" + resource_id + " .js-flag-actions").first().html(button).foundation();
     }
   };
 }).call(this);