Add funcitonality to flag an invoice as refunded

This will for example remove a conference registration automatically,
but most importantly it let's us keep track of our actual invoice
payments properly.
Add a verification prompt before deleting an invoice
Control invoice highlighting with CSS

Makes the highlight of deleted or unpaid/expired invoices be set using
CSS instead of hardcoded. Also, make the expired invoices show a bit less
"in your face" red, by just highlighting the date field. Makes the long
list of expired invoices that show up during conference registration a
lot nicer to look at.