Ref T6881. This adds the worker, and a script to make it easier to test. It doesn't actually invoice anything.

I'm intentionally allowing the script to double-bill since it makes testing way easier (by letting you bill the same period over and over again), and provides a tool for recovery if billing screws up.

(This diff isn't very interesting, just trying to avoid a 5K-line diff at the end.)

Used bin/phortune invoice ... to get the worker to print out some date ranges which it would theoretically invoice.