R1:0195e751c64d

Support basic ad-hoc invoices in Phortune

Summary:
This allows a merchant to send a user an invoice for something arbitrary, like services rendered.

Two major missing parts:

- These don't actually get marked as invoices. I'll fix that in the next diff, but it's not entirely trivial because `subscriptionPHID` is currently overloaded to also mean "is invoice".
- We don't send email automatically. I don't plan to fix that for now, since all our invoicing needs are covered by personal email.

Test Plan…