Add hide button in the user interface

The little menu with the hide notification link and block user link
has been added to each proposal notification.

JS for adding the fade efect has also been added.

app/assets/javascripts/application.js

@@ -33,6 +33,7 @@
 //= require moderator_comment
 //= require moderator_debates
 //= require moderator_proposals
+//= require moderator_proposal_notifications
 //= require prevent_double_submission
 //= require gettext
 //= require annotator