Create new Amplify user

[VincentPheng]

After a user signs up with the required information from the sign up form, create the user in Amplify and show them a temporary page that tells them to come back later after their account has been verified.

See: https://docs.amplify.aws/lib/auth/emailpassword/q/platform/js/#sign-up

EXTRA: Add relevant error messages when certain fields are given invalid data, i.e. "Invalid phone number", "Invalid email", "Email is already in use", etc.