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.

This allows the total discount of an offer to be capped.  It's a
slightly odd mechanism as the offer only becomes inactive once the cap
is exceeded, and it's possible that it is exceeded by a lot.

However, it's still a useful construct for managing offers.

Required several schema changes:
* Add num_applications to offer model to track number of times an offer
  has been applied within an order.
* Add max_global_applications to offer model to set a limit on how many times an offer
  can be used.
* Add frequency to order discount to provide insight into how many times
  an offer was applied for a particular order.

Structure of dashboard changes to talk about offer 'availability'
instead of assuming a start- and end-date.

They weren't applying to lines properly.  Fixed with test.