Add support for managing large amounts of vouchers grouped in sets.

This will simplify to filter out order lines, based on partner permission, without rewritting view code. By default, partners see all lines of the order, without respect whether lines were supplied by them or not.
Which may not always be the case, when we need partner to see only own lines.

phone_number_fields is more correct grammatically.

Add to the dosctring for the class, and display it in the documentation.

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- Remove outdated material relating to old versions of Django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.

Try to make it clearer how these are different from Attributes. Resolves #1776.

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.

Currently, a product's stockrecords are used to decide if a user is
allowed to edit a product or not. That doesn't work for parent products,
because they don't have stockrecords. So it's not possible to decide
whether a user is allowed to edit a parent product (and e.g. create
children) for it.

As far as I'm aware, this is not a regression of the
permission-dashboard; I don't think it ever worked. It just hasn't been
spotted because nobody needed it.

This commit also adds some tests to ensure this is enforced.

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.

The howto recipe on shipping has been rewritten to describe the new
changes to shipping functionality. The old shipping app reference has
been merged in too to avoid duplication.

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.