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
