Reimplement Sinch SMS validation

[wvdk]

We took this out of the signup flow because it didn't provide any value - users still had to sign up with their email.

Until we get to building out our own signup system which can use phone numbers without the need for email I suggest simply putting an "Add Phone Number" option to the UserSettingsVC. We can work on adding SMS only features in the future.

Attached is the initial proof of concept code - it should provide a good starting point:
Sinch SMS Validation Code.zip