Switch to using OmniAuth for authentication

This series of commits replaces our existing open_id_authentication + restful_authentication auth system with OmniAuth and a tiny bit of glue for managing sessions. Prior to this change, authentication was entirely broken on the master branch, due to Rails upgrades.

Currently, users can sign in via OpenID or Persona, but new authentication providers can be easily added. A database migration transfers OpenID identifiers for existing users to the new system.

The OpenID login form in this PR doesn't include the javascript openid-selector that we'd been using previously. This should be added before launching the osbridge CFP, using the newest version of that code or something similar, as tracked by #57.

Fixes #42.