attendees report says attendees have registered if they have *any* invoices, not just paid invoices

[chrisjrn]

The crux is that this bit of code doesn't actually filter as expected:

    attendees = attendees.annotate(
        has_registered=Count(
            Q(user__invoice__status=commerce.Invoice.STATUS_PAID)
        ),
    )

Instead, it just counts all invoices attached to a user.

[chrisjrn]

pyconau2017@731eee0#diff-890862ee2c6c4496e36619210f705073

[jamezpolley]

Seems to be a dupe of #123