Feature: who can set as admin when editing a user

Made changes to users_controller, users.rb and roles.rb
--> When editing a profile, only an admin can set or remove the admin role
--> Users who are not an admin aren't able to set themselves as admins
--> On sign-up, users can only elect to be a parent or a professional
--> if a user does not select a role, the role 'parent' is set as default