* 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

Change 'currency_digits': "accounting" to 'format_type': "accounting".

* Document how to customize dashboard forms. Thanks @okfish for providing instruction.

* Fix documentation for `OSCAR_INITIAL_LINE_STATUS`.
Setting value should be defained in the `OSCAR_LINE_STATUS_PIPELINE`, not `OSCAR_ORDER_STATUS_PIPELINE`.

- Remove outdated material relating to old versions of Django
- State that Oscar assumes that the email field is unique
- Update a number of links to Django documentation.
- Minor fixes to other bits of documentation.

Since there is no way to override SlugField and AutoSlugField in existing Oscar models, it was decided to pass `allow_unicode` parameter if this setting enable. This allows to achieve backwards compatibility with Django versions under 1.9, when this parameter was not supported yet. If parameter specified in the field expliticly - we do not override it, which allow to control slug fields individually in a normal way. For the reasons, explained above, standard SlugField was replaced with a custom one with the single slight change - control over `allow_unicode` parameter by the mentioned setting.

This commit adds a bit of documention around the Elasticsearch support
introduced in #1572. It also shuffles around the handler names to be
more consistent.

This was an oversight that is now corrected.

This commit also switches to the more appropriate pattern_name variable
instead of reversing ourselves.

Building upon the previous commit, we now use ordered dicts for our
search facets.

This commit removes a few long deprecated bits in the docs and in
code. It also updates the
release notes to include those removals, and highlights what has been
deprecated in 0.8. And it fixes a Sphinx warning when building the docs.

It only prevents adding a product to a range; offers can be applied
nonetheless (despite the docs saying otherwise). The same functionality
can be achieved by overriding get_is_discountable, so it can be safely
removed.

It must be left over from more glorious times.

This is the squashed version of #1369.

Closes #1369.

Oscar by default deleted image files and thumbnails. That is usually
fine, but has been an issue when using remote storages like S3. This
change also makes the only remaining import from sorl optional, hence
paving the way to making it completely optional.

Fixes #1373.

Importing things in settings is discouraged. But until now,
OSCAR_SLUG_FUNCTION could only be set as a callable, which requires
importing. It's now been updated to use the import_string function.

Seems like a good practice to define them explicitly.

Cherry-picked from d755aae.

The getting started guide now explicitly mentions copying the "missing
image" image when working with a remote file storage. Was raised in
issue #1314.

Oscar is slowly adding better support for switching languages on the
fly. That also means that locales need to be switched.

The issue this commit tries to fix is that when switching languages, the
locale for rendering prices doesn't change, and hence prices are
formatted incorrectly.
Oscar used to require hardcoding the locale for prices via
OSCAR_CURRENCY_LOCALE. I'm not sure what the reasoning behind this was,
because as far as I'm aware, the locale can be auto-detected from the
current language. This not only reduces complexity, but nicely works
when switching languages.

This allows importing a class or attribute by the reasonably common
string notation, which is useful for e.g. settings. When using settings,
we should prefer absolute imports over using get_class, because that
limits us to modules in INSTALLED_APPS.

* It can now be specified in OSCAR_DASHBOARD_DEFAULT_ACCESS_FUNCTION
* nav.py and menu.py were split up so that Node can be dynamically
  loaded and hence overridden

I recently needed to alter the access function and realised it's not as
easy as I thought.

Allow using a different app than promotions as the homepage view.

- Expand the ref/apps docs. (Still lots to do here.)
- Split up the index page links for clarity.

Added to release notes & settings reference.