hash tokens in database

implementation for discussion in #80

- add cookie_secret_file
- store cookie_secret on disk (or env or config file), instead of in database
- hash tokens in database (uses PasswordType from SQLAlchemy-Utils)
- don't persist configproxy token in db (restarting Hub requires restarting proxy, unless token is stored in env or config)
Arg. I'm this close to dropping Python 2 support in the Hub.