Very slow Sidekiq issue with large queue due to massive numbers of unread user notifications

I actually think we ought to have a default on setting here, there is almost zero value in a single user having more than 10,000 notifications, we should just purge anything beyond 10,000 out of the box

@codinghorror you happy with this ?

There is arguably value in capping unread at an even lower value like say 1000 per user, but the strong notification cap would completely resolve the issue experienced here anyway