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.

Fixes #376

This is a major clean-up of offers, tidying up the implementations and
rewriting the test suite.  This should also clean up some edge cases
around the use of max_affected_items.