Accepts an invitation, removing it and adding the invited user to the teachers or students (as appropriate) of the specified course. Only the invited user may accept an invitation. This method returns the following error codes: * PERMISSION_DENIED if the requesting user is not permitted to accept the requested invitation or for access errors. * FAILED_PRECONDITION for the following request errors: * CourseMemberLimitReached * CourseNotModifiable * CourseTeacherLimitReached * UserGroupsMembershipLimitReached * NOT_FOUND if no invitation exists with the requested ID.