Moves ENABLE_INSTRUCTOR_EMAIL and REQUIRE_COURSE_EMAIL_AUTH from settings files
to admin-accessible configuration models. This allows for the bulk email settings
to be modified without a new AMI deploy. See TNL-4504.
Also updates tests:
-python tests mock out the new configurations in place of the old settings
-lettuce test has been moved to bokchoy
(note that there was some loss of coverage here - the lettuce tests had
been doing some voodoo to allow for cross-process inspection of emails
messages being "sent" by the server, from the client! In discussion with
testeng, this seems outside the realm of a visual acceptance test. So,
the bokchoy test simply confirm the successful queueing of the message,
and leaves the validation of sending messages to the relevant unit tests.)
-bok choy fixture has been added, to replace the settings in acceptance.py
-lettuce and bok choy databases have been updated to reflect the backend changes
The new default is to have bulk_email disabled, we'll need to call this out in the
next OpenEdx release to ensure administrators enable this feature if needed.
