They aren't treated specially any more.  They become part of the
'applications' collection rather than being attached to the basket.

This paves the way for custom benefits to return successful applications
that don't lead to a discount.

This makes it more efficient to load offers now that we can't simply use
the date filters to exlucde unavailable offers.

The is_active field has been reworked to depend on the
get_max_applications method.

The save method of ConditionalOffer now auto-sets status.

Fixes #403

This might be futile.