- Replace use of deprecated `request.is_ajax()` with equivalent.
- Replace use of `imp.reload` with `importlib.reload`

Co-authored-by: jrymer <jrymer@dommarket.fr>

- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

Also fix "is_anonymous" attribute value in WebTestCase subclass.

Co-authored-by: Basil Dubyk <samitnuk.work@gmail.com>
Co-authored-by: Alexander Gaevsky <sasha@sasha0.ru>
Co-authored-by: Joseph Wayodi <jwayodi@gmail.com>

Instead of checking for truthiness we should check for equality
(because any status code is `True`).

`override_settings` used instead of updating setting in `setUp`/`tearDown`.

Currently, if a valid basket line is marked for deletion, any other
invalid basket lines on the formset prevent it from being deleted.

Regression introduced in cca43aa4d1

So that migrations are not required for changes to the
"OSCAR_IMAGE_FOLDER" setting, for projects that have not forked the
"catalogue" app.

Update the TestDashboardIndexForAnonUser test case

Also refactor catalogue option group and option tests.

- Move Oscar app configs into Django app config classes
- Rename Oscar/Django apps' app config module
- Remove "oscar.get_core_apps()" and its app overriding
- Remove "oscar.OSCAR_CORE_APPS"
- Port app forking code to work with Django app registry
- Port class loading code to use app configs to get app properties

Fixes #2579.

See #1674.

Pass communication event type code to OrderPlacementMixin.get_message_context()