Some new Sign In options on /ftu-3

* Guest Mode button in Online mode (should open app in offline mode)
* Changed some button labels and text

Vaguely matches design in:
https://github.com/mozillafordevelopment/webmaker-app/issues/350

@k88hudson Questions:
* Do we have a way to start the app in offline mode / guest mode even when you have internet? 
* Can we detect if we're online and show the appropriate options on ftu-3