Allow org managers to add users outside of org

[richard-cox]

• At the moment org managers cannot add roles (like user) to users who don't already have roles in that org.
• This is because non-admins cannot fetch users via /users, which I can kind of understand. So for non-admins we fetch users via org level requests which only return users with roles.... leading to only users with org roles presented in org users list.
• It's possible for non-admins to add users to an org by username, as long a ff set_roles_by_username is set
• We should add a new stepper that allows org managers to add org user role to user's with usernames they supply
• Raised by akp in slack

[richard-cox]

This feature would also help CF Admins at the CF level when there are too many users to show in the user's list. Raised by Hillary Jeffrey in slack, along with original issue.