Ensure deleting a site instance doesn't delete related orders
Remove uploaded media when rebuilding sandbox
Align voucher discounts correctly on basket page
Revert to using normal date fields in dashboard voucher form

There was a strange conflict between the field and widget formats, where
they fell out of sync and the submitting data was never valid.
Adjust how voucher additions are tracked

The old receiver wasn't hooked in, and also wasn't counting correctly.

Fixes #829
Update voucher addition counter when voucher is removed
Minor adjustments to voucher dashboard templates