Add support for sponsors requesting explicit contract at click-through levels

Add support for sponsors requesting explicit contract at click-through levels

Click-through contracts makes it easier for most, but some sponsor
organisations require an actual contract at these levels as well. To
handle this, add an option during contract/address verification to
request an explicit contract, which can then be either manual or
digital, and will both prevent the clickthrough contract and invoice to
be sent until thsi contract has been completed.