Add user authentication

[JakeWharton]

This allows

• Grabbing the user photo for the nav drawer.
• Starring and unstarring repos from the app.
• Concrete demonstration of contextual debug actions.

[raskasa]

Do you have specific examples of contextual debug actions that you would want to show for a "Logged In" user?

cc @mattprecious

[JakeWharton]

The action is of logging in on the sign in screen itself.

[mattprecious]

Yes, this would be a mock-mode specific action that would just enter sane data to exercise the UI and get you through the screen with as few taps as possible.