Adds cohorts as valid bulk email targets.

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.

By default, disable all caching in tests, to preserve test independence.
In order to enable caching, inherit from CacheSetupMixin, and specify
which cache configuration is needed.

[EV-32]

TNL-4193

TEST_DATA_MIXED_MODULESTORE
Remove these test mixed modulestores:
TEST_DATA_MIXED_TOY_MODULESTORE
TEST_DATA_MIXED_CLOSED_MODULESTORE
TEST_DATA_MIXED_GRADED_MODULESTORE

* truncate email from addresses if >320 chars when encoded (TNL-4264)

* use exact lengths

TEST_DATA_MIXED_MODULESTORE
Remove these test mixed modulestores:
TEST_DATA_MIXED_TOY_MODULESTORE
TEST_DATA_MIXED_CLOSED_MODULESTORE
TEST_DATA_MIXED_GRADED_MODULESTORE

clean up and fix quality violations

since get_course can never return None, we can remove these lines

clean up and fix quality violations

since get_course can never return None, we can remove these lines

TNL-2677

TNL-1877

This reverts commit 228cca4e38ec79f5cbb54098c788ab9556f602e2.

This reverts commit c2e5bd3d.

Update edx-lint to the version that checks if tearDown uses super.

Convert a number of tearDown methods into addCleanup.

Remove some unneeded tearDown methods: no need to call patch.stopall if
none of them were started with patch.start.

The existing pattern of using `override_settings(MODULESTORE=...)` prevented
us from having more than one layer of subclassing in modulestore tests.

In a structure like:

    @override_settings(MODULESTORE=store_a)
    class BaseTestCase(ModuleStoreTestCase):
        def setUp(self):
            # use store

    @override_settings(MODULESTORE=store_b)
    class ChildTestCase(BaseTestCase):
        def setUp(self):
            # use store

In this case, the store actions performed in `BaseTestCase` on behalf of
`ChildTestCase` would still use `store_a`, even though the `ChildTestCase`
had specified to use `store_b`. This is because the `override_settings`
decorator would be the innermost wrapper around the `BaseTestCase.setUp` method,
no matter what `ChildTestCase` does.

To remedy this, we move the call to `override_settings` into the
`ModuleStoreTestCase.setUp` method, and use a cleanup to remove the override.
Subclasses can just defined the `MODULESTORE` class attribute to specify which
modulestore to use _for the entire `setUp` chain_.

[PLAT-419]

Move modulestore config for tests to an importable location
Disable pylnt warning for lms imports in common tests
Refactor all testcases that loaded all xml courses
TE-610
TE-489