Use normalize_username instead of .lower

allows for following normalization changes

Author: minrk

firstuseauthenticator/firstuseauthenticator.py

@@ -138,7 +138,7 @@ def validate_username(self, name):
 
     @gen.coroutine
     def authenticate(self, handler, data):
-        username = data['username'].lower()
+        username = self.normalize_username(data['username'])
 
         if not self.create_users:
             if not self._user_exists(username):