Option to resend SMS and/or change signin phone number

[devonzuegel]

When a user goes through the sign in flow, they are sent an SMS message.

This may fail for some reason, so we should give them the option to:

1. resend the SMS, and
2. change their phone number if they're unable to make it work.

Note: We should throttle how quickly they can resend the code, because sometimes they just need to wait a few seconds for the message to arrive. Maybe include a countdown timer so they have a sense of how long they should expect to wait before retrying.