Create onboarding sign-in page

[rock3r]

The first onboarding page is the sign-in page; it should look something along these lines:

Please refer to @ScribblyPixels' designs.

The "no thanks" button brings users to the next page, "yes please" starts the sign-in process. If sign-in succeeds they're immediately brought to the next onboarding page, otherwise they remain on the current page and some error is displayer (design TBD).

⚠️ The design for the error state is TBD and the existing designs are still WIP