CLC-3646 - Implemented cacheing of result for Canvas User ID for increased performance

Moved #user_currently_teaching? method to Canvas::CurrentTeacher
Added #can_create_course_site? to Canvas::PublicAuthorizer with direct result for Canvas User ID.
Updated dependency in User::AuthPolicy#can_create_canvas_course_site?

https://jira.ets.berkeley.edu/jira/browse/CLC-3646