Add dates to the billable users HubSpot sync where the count resets for multi-year deals

[mkdir-washington-edu]

This is follow-up work to the tasks on the board here: https://github.com/orgs/hypothesis/projects/139/views/2 The main request: https://github.com/hypothesis/product-backlog/issues/1536

The work already done fits the current use cases:

• For a closed/won deal less than a year (example: Jan 1 2000 through July 31 2000) the current system is ideal; monthly the billable user count stays the same or goes up, re-calculated every month
• For a closed/won deal equal to one year (example: Aug 1 2000 through July 31 2001) the current system is ideal; monthly the billable user count stays the same or goes up, re-calculated every month

For deals longer than a year, it's helpful to know billable usage per year, since getting the renewal of a contract typically depends on the usage within the last full year of the contract. To see this accurately, billable usage should reset to 0 in a way that leads to tracking a years worth of usage leading to the contract end.

For the following use cases, billable users will need to reset to zero once a year and then start growing again

• For a closed/won deal of 1.5 years, 2.5 years, or similar (example: Jan 1 2000 through July 31 2002) we need to set billable users back to 0 every August 1. That way usage will always grow as we get close to July 31st in every contract year.
• For a closed/won deal of 2, 3, or more years, (example, Aug 1 2000 to July 31 2002) we need to set billable users back to 0 every August 1.

[mkdir-washington-edu]

We should review this at the next support/engineer sync, and then icebox it until November or so. This work isn't needed until January 2025 at the earliest.