Implement bulk payment for conference registrations

Regular users register the normal way, but they never proceed to payment.
Instead, the "bulk payer" will enter their email addresses on a form, confirm
all the payments, and get a single invoice with all of them on it.

The invoicing is integrated with the general invoice system, so once the invoice
is paid, it will automatically flag all the individual registrations as
confirmed as well. (Same goes for cancelling which will unlock them all)
Send manual confreg emails using the new mail queue system

Makes it behave much nicer when there are issues sending, and doesn't
end up calling out to SMTP from within a regular web request.
Better/more complete instructions for bulk payments
Make the registration details of finished registrations look nicer

This was made to be styled by someone with more style skills than me, but
seems nobody is around with the time to do that. So clean it up a little
bit at least, making it not suck. More work can surely be done on it.
Don't show conference information section if there are no fields in it