Implement backend for pairing Briar with mailbox

The backend should accept an onion address and a single-use token from the UI and use them to pair with the mailbox. It should return status updates to the UI. If pairing is successful, the onion address and long-term auth token should be stored in the Briar database.

Depends on briar-mailbox#3 (closed), briar-mailbox#50 (closed), #2169 (closed).