R1:f0c6ee48233a

Add "Contact Numbers" so we can send users SMS mesages

Summary:
Ref T920. To send you SMS messages, we need to know your phone number.

This adds bare-bone basics (transactions, storage, editor, etc).

From here:

**Disabling Numbers**: I'll let you disable numbers in an upcoming diff.

**Primary Number**: I think I'm just going to let you pick a number as "primary", similar to how email works. We could imagine a world where you have one "MFA" number and one "notifications" number, but this seems unlikely…