Use 'developer' OmniAuth strategy by default

- Turn on 'identity' OmniAuth strategy in config.ru
- Add more info fields to identities table
- Turn off 'nosniff' header as workaround for OmniAuth::Form bug
- remove signup actions in favor of identity registration through
  OmniAuth
- create User and Authentication after creating an Identity
- add Identity#persisted? to make identity registration work properly