Disallow registration of the same phone number twice

[woksin]

This is especially important for data collectors. If two different data collectors is registered with a similar phone number we get undefined behavior in user management and volunteer reporting BCs.

Perform command validation in backend to ensure that no phone number is registered twice for data collectors.

We need to store the phone numbers somehow, a separate Mongo collection? Or perhaps it deserves its own aggregate. Thoughts, @einari , @Velcrow81 ?