Convert instructor email messages to ACE

...est_site/templates/emails/enroll_email_allowedmessage.txt

...est_site/templates/emails/enroll_email_allowedsubject.txt

...st_site/templates/emails/enroll_email_enrolledmessage.txt

...st_site/templates/emails/enroll_email_enrolledsubject.txt

...t_site/templates/emails/unenroll_email_allowedmessage.txt

..._site/templates/emails/unenroll_email_enrolledmessage.txt

...tes/test_site/templates/emails/unenroll_email_subject.txt

lms/djangoapps/ccx/tests/utils.py

lms/djangoapps/instructor/enrollment.py

lms/djangoapps/instructor/message_types.py

lms/djangoapps/instructor/tests/test_access.py

lms/djangoapps/instructor/tests/test_api.py

...jangoapps/instructor/tests/test_api_email_localization.py

lms/djangoapps/instructor/tests/test_enrollment.py

lms/djangoapps/instructor/views/api.py

lms/djangoapps/instructor/views/registration_codes.py

lms/templates/ccx/enroll_email_allowedmessage.txt

lms/templates/ccx/enroll_email_allowedsubject.txt

lms/templates/ccx/enroll_email_enrolledmessage.txt

lms/templates/ccx/enroll_email_enrolledsubject.txt