Moved new group user to its own action GroupsController#new_users.

git-svn-id: http://svn.redmine.org/redmine/trunk@13602 e93f8b46-1217-0410-a6f0-8f06a7374b81
Adds tests for new routes.

git-svn-id: http://svn.redmine.org/redmine/trunk@13603 e93f8b46-1217-0410-a6f0-8f06a7374b81