- Add new "DEFAULT_AUTO_FIELD" Django 3.2 setting
- Fix deprecated (in "gettext") use of non-int numbers for selecting
  plural translations
- Provide "get_response" callable when creating middleware-factory
  instance in tests

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>

Also remove unused migrations test for MySQL.

* Implement basic abstract thumbnails generator backend

* Create `oscar_thumbnail` template tag in `image_tags`

* Add `easy_thumbnails` to `INSTALLED_APPS` in sandbox settings

* Modify `primary_image` method of `Product` model

* Override in templates Sorl's `thumbnail` tag with `oscar_thumbnail` tag

* Check that related module is installed during thumbnail class initialization

* Add params to `original` field of `ProductImageFactory`

* Add media related URLs to test settings

* Add unit tests for thumbnailer backend

* Create test helpers in `test/utils`

These helpers will be used in tests related to thumbnails

* Add tests to check `oscar_thumbnail` template tag

* Add test to check images/thumbnails deleted for deleted product

* Move `sorl-thumbnail` and `easy-thumbnails` to `extras_require`

With this change these packages can be installed as
* `pip install django-oscar[sorl-thumbnail]`
* `pip install django-oscar[easy-thumbnails]`

* Updated "Template tags" section in docs with info about `oscar_thumbnail`

* Describe `OSCAR_THUMBNAILER` setting in "Settings" section of the docs

* Add information about created thumbnail backend to the release notes

- 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

See https://code.djangoproject.com/ticket/25120

Django 1.9 introduced the AUTH_PASSWORD_VALIDATORS setting which allows for
more flexible password validation. Switch to using that when it is available,
with a fallback to Oscar's previous behaviour in Django 1.8 or when this
setting is empty.

`DEBUG_TOOLBAR_PATCH_SETTINGS` is obsolete in django-debug-toolbar 1.6. We also don't need to specify `DEBUG_TOOLBAR_PANELS` as it is identical to the default.

This fixes a DeprecationWarning

Results in too much clutter in Sentry

This allows the latest.oscarcommerce.com to run with memcache

This sets the Content-Length response header which is required for
CloudFront and possible other cdn's

Since there is no demo or us site anymore we can simplify the structure
somewhat by moving the sandbox one directory up

Example sites now sit in their own folder (sites) for easier
organisation.

This allows members of the public to play around with the dashboard
site.

I'm sick of getting the error emails.

This view has a form that assumes anon checkout is allowed, which it
isn't always.  Treating it as a special case was causing it to be
displayed when it shouldn't.

This makes it much easier to get started when contributing.  The docs
are also updated.

This makes it much easier to get started when contributing.  The docs
are also updated.