Phone number integration

[Ashoat]

When a user creates an account, they should be prompted to provide their phone number, and we should send a text to confirm it. This is crucial for getting contact list sync (#26) to work.

I'm not sure about what details we should ask for in the registration screen. Right now we ask for username, email, and password. We could pare this down to just username / phone number, or we could leave phone number to be specified in the NUX (#21).