Set up everything for enabling number verification

This puts lots of the needed code in place to actually begin doing
number verification. It doesn't send out a signup text yet. Getting
there.

To enable it, just remove the comments in /Routes/subscribers.php
and the call to setVeried().
Send outgoing message on signup

So when you subscribe, you'll instantly get an SMS with
instructions.
Add unsubscribe capability

So if an incoming message comes in with the word "unsubscribe"
in it, drop them from the subscribers table.