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:
Merchants have a new "new invoice" option:

This leads to selecting a user and account, and then you can generate the invoice (only one actual "purchase" / line item for the moment). You can add a longer-form remarkup description to contextualize the billable items:

This sends the invoice and takes you to the merchant order overview screen:

For now, you copy/paste that link into a nice personal enterprisey business-to-business email; the recipient sees this:

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D12478