move to firebase

[djay]

@waf Had some good discussions today
Main advantages seem to be

• No more being rate limited
• Free tier means people can still fork it and setup easily
• No more extra server for oauth but still works with github oauth
• pubsub builtin so get instant updates. no more polling
• no more trying to bend our model to fit github. more structure data

Cons

• Can't reuse discussions and email notifications. But perhaps we can still link to github for bigger projects?