Add `Enable(Push)Notifications` step to initial Onboarding flow

This is more specific and dependent on mobile phones, but something we will want to encourage, so I think it's good to start considering it as we get closer to an .apk. Lower priority, but a feature we'll likely encourage nonetheless.

It could be part of a 2 or 3 step onboarding flow (Enable Notif, Recovery Phrase, Passcode, etc..).

Still needs a bit of design work, but ....