Fixes #376

* Fixes bug with prefetch_related where looking up secondary relations
  raises a DoesNotExist exception.
* Reworks cached partner map to use PKs instead of names.
  Availability logic from product class is looked up in product obj
  rather than in wrapper to avoid unnecessary queries.

It's potentially confusing to have it default to a value as you might
miss this in your project.

This involves:
* Better checkout templates
* Updated availability logic for products with no stock
* Updated dashboard templates

Turns out this is a bit confusing.  Better for it to do nothing by
default and this behaviour can be enabled when it is needed.

Also split them into unit and functional tests.