Checking uniqueness of unconfirmed number

Issue Summary

Right now, one cannot enter a mobile number if it has already been registered on the site, even if the existing mobile number is unconfirmed. This doesn't really make sense, as anyone can input anyone else's mobile number and prevent the real owner from using their own numbers.

There needs to be two changes
1. When inputting a new mobile number, it should be checked for uniqueness only against confirmed numbers.
2. When confirming a mobile number, it should be checked for uniqueness against other confirmed numbers

(2) is necessary because (1) allows the same number to be unconfirmed, and in the very small possibility that the separate user accounts both have access to the same mobile phone number.