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.

- Remove outdated material relating to old versions of Django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.

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.

Done as it became apparent in #968 that further clarification is needed.

We currently mention it in internals/sandbox.rst. I've added a note to
the dashboard app (ref/apps/dashboard.rst), the sandbox/urls.py and
expanded on the feature explanation on index.rst.

Fixes #816

Some of them are rather basic, but it's a start.