instructorCourseAddInstructor: vulnerable to eventual consistency #2474

For Issue #2474

Registration email method now uses the previously created instructorAttributes instead of trying to query for the same thing. Manually tested on staging server to ensure that emails are still sent out correctly and the instructor can join the course properly.

p.s Somehow github lumped commits from my other branch together with this branch, but I already did another commit to remove those changes.
Is there another method this one is replacing? In that case, see if that method can be removed.