Kind of generate a bill for users

Summary:
Ref T6881. This generates a product, purchase and invoice for users, and there's sort of some UI for them. Stuff it doesn't do yet:

• Try to autobill when we have a CC;
• actually tell the user they should pay it;
• ask the application for anything like "how much should we charge", or tell the application anything like "the user paid".

However, these work:

• You can technically pay the invoices.
• You can see the invoices you paid in the past.

Test Plan: Used bin/phriction invoice to double-bill myself over and over again. Paid one of the invoices.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6881

Differential Revision: https://secure.phabricator.com/D11580