R1:d1e793a292a1

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.