🔨 To-Do: Finish phone authentication

[sirmammingtonham]

Either finish or decide to scrap phone authentication.

Finishing requires setting up Android and iOS permissions to enable safetynet and silent notifications respectively (https://firebase.google.com/docs/auth/android/phone-auth, https://firebase.google.com/docs/auth/ios/phone-auth).

Scrapping the feature means we will assume that users will not attempt to add wrong phone numbers to troll, or at least that we think few enough ppl will attempt this so the added authentication is overkill. In this case, we go back to requiring phone number input on the sign up page and store phone numbers in the firestore database.

Phone numbers are needed so saferide drivers can contact users quickly.