Log in as admin
add a username with a dot "hans.muster", fill all other fields
click on "create user" -> nothing happens
-> better to show the same warning as when a user can sign up:
"Username - User name must only contain alphanumeric characters, underscore and dash"
Further problem: when a user is auto-detected (after a commit) a dot is allowed, and in our case wanted.
-> perhabs it is better to allow dots