Cannot choose a different username for a different name service URL

Issue generated from Tuleap's migration script. Originally submitted by: Maxim Cournoyer (mcournoyer)

* Environment:
GNU/Linux Ubuntu 16.04 with the Ring Gnome client

* Reproduce steps:

1. Go to settings.
2. Create a new account with a registered public username.
3. In the "General" tab of the newly created account, change the name service URL to something else (it doesn't need to be an existing URL).
4. Try changing the username.

* Expected result:
When the user edits the "name service URL" field, the UI should allow the user to change its username to a different one, and allow for a new username registration to happen on that different name service URL.

* Actual result:
The username cannot be changed: the user cannot choose (let alone register) a different username for a (potentially) different name service.