Only use is_successful for dashboard row highlighting
Tweak template context for preview view in sandbox

To make things clearer, we now pass in the bankcard instance rather than
grabbing it off the bankcard form within the template.
Handle empty audit objects better
Add support for voiding transactions
Ensure the (shipping|billing) addresses are passed to the facade
Remove uniqueness constraint on vendor_tx_code

As VOIDs re-use the same code.