Sign up: Username changer loading shouldn't be a modal

When the username list loads, it shouldn't use a modal view with the loader, as the above.

The change could be done at two levels.

Level 1

We don't show anything while loading: the list is pre-populated with the existing username (thus, it has at least one item, selected) while loads the first time, and for loads while searching the list simply updates once the API is returned.

Level 2

If we want explicit indicators, here's a design for it: