usernames: clarify username registration states

Issue generated from Tuleap's migration script. Originally submitted by: Isak Rubin (isakrubin)

username and avatar seems to not be synced across devices.

* Environment:
Android 7.1
Debian Stretch

* Reproduce steps:
Install on android, register account, setup avatar and username.
Install on debian using deb package, connect account using pincode.

* Expected result:
Expect my username and avatar to be available on my debian setup.

* Actual result:
Avatar and username not synced, not even able to set my username in the settings as it says it's not available.