Move auth handlers, sessions, gates, checks into airlock project

The code that handles webapp2 sessions, request handling, request-level security, etc. can be abstracted into a separate library for maintenance and re-use.
- https://github.com/grow/jetway/tree/master/jetway/auth
- https://github.com/grow/jetway/blob/master/jetway/main.py#L61

https://github.com/grow/airlock
