User.publish_notifications_state is called every time a notification is
created, this can become a very critical code path.
On some heavy notification related sites this can be a major CPU user on PG
This index makes it much cheaper to publish notification state, cause a
simple index lookup does the trick.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
