Reduce scope of `UberAuthenticating` protocol

[edjiang]

Previously, the UberAuthenticating objects kept track of the authentication workflow, saved tokens to the Keychain, etc.

Since that behavior was moved into the LoginManager in #200, this change reduces the UberAuthenticating protocol to just two functions:

• The login URL to redirect the end user to
• Consuming the response after the user is redirected back to the app.

This allows us to match different forms of authentication (implicit, native, etc) with different ways of presenting it (deeplinks, SFAuthenticationSession, SFSafariViewController, and just Safari) a lot easier.

[bobz]

Unfortunate that github isn't picking up the renames here.