Allow for Proxy Number sharing.

[seanhealy]

The biggest fixed cost of running this are the proxy numbers. These are also very shareable. Since it's only LightPhone users who will be sending messages to these proxy numbers we can identify the contact that they are proxying to by correlating the proxy number to the user who's light phone sent the message. Likewise we can do the opposite for the other message direction.

There would be other considerations when we get to here at any scale such as passing on the messaging costs but at a low scale that could be handled with donations or with some other ad-hoc mechanism.