Don't render blank table in categories dashboard
Improve catalogue admin for options
Add fixture for creating a variant product

And load it for the sandbox by default.
Extend strategy class to work with variant products

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.
Ensure that only available variants are in basket dropdown
Use a custom widget to show unavailable variants in dropdown

We define a custom select widget which allows certain choices to be
marked as disabled.  We use this to include unavailable variants in the
add-to-basket dropdown.
Merge remote-tracking branch 'upstream/issue/481'