Web Push support (experimental)

This version adds an experimental implementation of web-push to the generic push-notifications plugin using this
commit (d7f0f9f)

The changes involve checking the configured app service domain. If it matches the XMPP server domain, then it assumes that the push notification is a web-push notification.

It then extracts the JSON subscription details from the saved pushOptions, encodes the delivery payload and tries to send it directly to the browser vendor push service with an HTTP POST as specified in the web push specification