CLC-4298 - Added Canvas::CourseAddUser.granting_roles method which returns roles which users may grant to other users
CLC-4298 - Added support to CanvasCourseAddUserController#course_user_roles to return grantingRoles defining roles which the user may grant to others
CLC-4298 - Moved  calls from AngularJS controller to canvasCourseAddUserFactory.js
CLC-4298 - Updated 'Find a Person to Add' tool to use authorized user roles array
CLC-4298 - Added authorization check for role specified in user enrollment request
CLC-4298 - Updated specs to use new format for stubbing
Merge pull request #2994 from redconfetti/CLC-4298-ta-security-fix

CLC-4298 - TA role can add a Teacher using "Find a Person to Add"