Fix #5376 - Ensure links have correct ID

This ensures that links to comments have the correct ID (the
ID of the Comment object) as well as fixing deletion on reports
and fixing a few other little things. I hope this fixes the #5376
issue, though cannot reproduce so unable to confirm.

mediagoblin/notifications/__init__.py

@@ -75,6 +75,11 @@ def mark_comment_notification_seen(comment_id, user):
         obj_pk=comment.id,
         model_type=comment.__tablename__
     ).first()
+
+    # If there is no GMR, there is no notification
+    if comment_gmr == None:
+        return
+
     notification = Notification.query.filter_by(
         user_id=user.id,
         object_id=comment_gmr.id