TNL-4356

Allows multiple bulk email targets to be specified at once.

-The previous "All" option has been split into "Staff" and "Learners"
-The backend changes made here lay the groundwork for cohort emailing
-The data migration, 0005, is somewhat large and requires deploy attention
-Tests have been updated
-Numerous safe-commit-linter fixes are included

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.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

Add the ability for Microsites to be able to send appropriately branded emails rather than the default system installed version

fix tests. Update Admin views/forms.

enforce uniqueness constraint in the Admin ModelForm submissions

Update bok_choy cached DBs with new schema changes

PR feedback

LMS-2134

Adds the edX Marketing-approved template as html default.

Includes Data + Schema migrations for optout email -> user.id change.
Note that migrations should be reversible.

Switch to using decorators; refactor and cleanup tests.

Adds a new Email link to the instructor dashboard for frontend interface to send
email to course members. Adds a feature flag ENABLE_INSTRUCTOR_EMAIL to toggle this.
Creates a new djangoapp bulk_email that handles this action by getting the recipient
list and batching the emails to different celery tasks to do the actual sending.
Requires lynx package to convert HTML email to plaintext. Handles SMTP errors by
retrying or falling through to the next email. Adds the option to opt out of course
specific emails in the user dashboard with an Email Settings link for each course.
Uses severable configurable settings with defaults. DEFAULT_BULK_FROM_EMAIL
specifies the from address for email. EMAILS_PER_TASK specifies the number of emails
each celery task takes on. EMAIL_HOST, EMAIL_PORT, EMAIL_HOST_USER,
EMAIL_HOST_PASSWORD, and EMAIL_USE_TLS for the SMTP email backend settings.

Co-authored-by: Akshay Jagadeesh <akjags@gmail.com>