* Do not cancel PuSH subscriptions after encountering "permanent" error response

After talking with MMN about it, turns out some servers/php setups do
return 4xx errors while rebooting, so this anti-feature that was meant
to take load off of the hub is doing more harm than good in terms of
breaking subscriptions

* Update delivery_worker.rb

* Framework for delivery worker spec

* Refactor of pubsub delivery worker

* Add specs for valid challenge response from pubsub confirmation worker

* Refactor the pubsub confirmation worker

to all subscribers. IDN-normalize callback URLs for subscriptions on insert.