Revert changes in #1913 and #2294 that required a stock record to have price_excl_tax set in order to report the product as available.

Add separate checks to the basket form/add logic that checks at the time of
adding a product to the basket whether a price exists, and report an error if it doesn't.

Fixes #2664.

Better to use filenames which are commonly used in the python word then
custom ones.

This is the result of wild grepping through the codebase. Nothing too
exciting to report, really. create_product now sets a products structure
to 'child' if a parent is specified.

Tests now pass and no deprecation warnings are thrown. Beautiful.

Oscar now supports setting a price of zero for products.

Please note that currently a price of zero has no other special-casing
apart in a template. Notably, setting a currency is still required and
you can't add products to a basket of a different currency. This can
be improved if needed.

In the templates, the product price is displayed as "Free" instead
of zero (and whatever currency).

Writing an (initially failing) test exposed slightly incorrect handling
of the passed price in the stockrecord factory, as a price of zero
would always be replaced by the default price.

Fixes #1347.

More explicit innit.

This allows availability to be correctly determined for complex basket
lines which use basket attributes to hold information vital for
availability decisions.  An example is bundle products that store bundle
product IDs in the basket line attributes.

A new template tag is also introduced, which has an inconsistent name
with the product version.  This will be fixed in a later commit.

The strategy API has gained a new method: fetch_for_group, which is for
fetching stock information about a group/parent/abstract product.  Like
the existing fetch method, this is highly customisable.

For a group product, we fetch all relevant variants and stockrecords; we
then pass these to the methods that determine availability and pricing.

The pricing display logic for a group product has been simplified a bit.
We now just look at the first stockrecord and use that price - we no
longer say "From £X".  This seems to be a more common pattern in the
wild and allows the templates to be simplfied as they no longer need to
handle group and standalone products separately.

This splits the logic nicely into separate classes that can be mixed
together to create the strategy that you want.