Add role new relation : roles_roles named assignable_roles :
 * this relations symbolise which roles (role_id) can be granted for a given role (assignable_by_role_id).

 E.g : ProjectManager can assign "TeamMember" and "ProjectManager" roles to members.
 But EngagementManager can assign "TeamMember", "ProjectManager" and "EngagementManager" roles to members.
Check if user can set/change members' role.