Give Phortune carts a proper isInvoice flag

Summary:
See some earlier discussion in D11593:

One thing I'm vaguely thinking about is the possibilty that users may be able to invoice one another directly, eventually. For example, we might invoice a contracting client.
We might need an isInvoice flag eventually, but subscriptionPHID is a reasonable stand-in for now.

This adds such a flag.

Test Plan:

• Generated an ad-hoc invoice and verified it showed up in the right place.
• Used bin/phortune invoice to invoice a subscription and verified it worked correctly.
• Paid an invoice and saw it leave "pending invoices" status.

{F377029}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12480