use urlgen and store it in a variable

mediagoblin/static/js/notifications.js

@@ -41,7 +41,7 @@ $(document).ready(function () {
         mark_all_comments_seen.onclick = function() {
             $.ajax({
                 type: 'GET',
-                url: '/notifications/comments/mark_all_seen/',
+                url: mark_all_comments_seen_url,
                 success: function(res, status, xhr) { window.location.reload(); },
             });
         }

mediagoblin/templates/mediagoblin/base.html

@@ -37,6 +37,9 @@
             src="{{ request.staticdirect('/js/header_dropdown.js') }}"></script>
     <script type="text/javascript"
             src="{{ request.staticdirect('/js/notifications.js') }}"></script>
+    <script>
+      var mark_all_comments_seen_url = "{{ request.urlgen('mediagoblin.notifications.mark_all_comment_notifications_seen') }}"
+    </script>
 
     {# For clarification, the difference between the extra_head.html template
      # and the head template hook is that the former should be used by