Support editing usernames

[sbddesign]

Currently, we do not have a way for a user to change their BIP-353 username after the fact. We should allow this.

Requirements

As a User

• I can login to Twelve Cash, find my existing usernames, and then choose to edit the username. From there, I can change the on-chain address and the BIP-21 properties, such as lno, sp, label, and LNURL Pay.
• After editing, I am informed that it may take some time for DNS propagation to occur.
• I learn that if I want to change the name itself, that is a new order (make a new random name or buy a new custom name).

As a Developer

• I can hit the API endpoint to update an existing username (all of the above but with a UI)