Allow restricting allowed usernames

Such as "funkwhale", "root", "admin", as those may be reserved on the application.

changed milestone to %0.8

mentioned in commit 4b69d64d

mentioned in merge request !107 (merged)

closed via commit 4b69d64d

closed via commit 952407d1

closed via merge request !107 (merged)

changed milestone to %0.8

mentioned in commit 952407d1

mentioned in commit fba9a5c9