* Retrieving product options now performs only 1 query.

Also it returns all the product options combined. This makes more sense because
as the existing tests prove, there is already a way to retrieve the product_class
options and the product options separately.

* 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

This change will allow to import `run_concurrently` helper for test
purposes in other libraries.

* enforce max_affected_items during offer application

also restrict the usage of offer consumption to line.consume()

* improve test coverage

Update the TestDashboardIndexForAnonUser test case

Add functionality to check if a voucher is
available for a basket or not.

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

This change fixes the inconsistency with range.all_products() which
didn't check for exclusions when includes_all_products is set

Every other bit of oscar expected range.includes_all_products means
that all products are included, no questions asked. The dashboard
interface even hides the option to edit/remove individual products.

The short circuiting in ConditionalOffer.products() is removed so
the range.all_products() logic is applied consistently.

Fixes #2579.

When `remaining_weight` is "0" (zero) we should not get band for it.