Replace bulk email settings with admin config models

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.

lms/envs/devstack.py

@@ -36,9 +36,6 @@ for log_name, log_level in LOG_OVERRIDES:
 ################################ EMAIL ########################################
 
 EMAIL_BACKEND = 'django.core.mail.backends.console.EmailBackend'
-FEATURES['ENABLE_INSTRUCTOR_EMAIL'] = True     # Enable email for all Studio courses
-FEATURES['REQUIRE_COURSE_EMAIL_AUTH'] = False  # Give all courses email (don't require django-admin perms)
-
 
 ########################## ANALYTICS TESTING ########################