R1:01c99335fdd7

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…