Enhanced Billing Timestamp Support
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Previously, billings had to make do with a single timestamp attempting
to fill two different roles.  In the case of an overdue fine, the
timestamp represented the *end* of the fine period for that billing,
while for all other fines, the timestamp was merely the time the bill
was created.  This setup generally worked, but not without confusion,
and limited our ability to understand and process the data.

Billings will now have up to three timestamps: a create date, and when
applicable, a fine period start and a fine period end.  This clarifies
and simplifies things like backdating, retrospective fine generation,
account balancing for negative balance avoidance, and billing timeline
views.