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.

So we add handling to the filter where we check the value can be
converted to a valid decimal.  If not, return an empty string.

Fixes #316

Plus fix a bug when handling None.