Fix Harvest to update project budget based on retainer

It should be possible to ‘fix’ Harvest using its own API:

Updating the project budget, above, when a retainer payment is recorded.

I’m not sure if we can get Harvest to notify our app when a payment is recorded in order to trigger the budget update, and that’s pretty essential to keep the code reasonable (polling and comparing regularly would be unreasonable, in my opinion).

Actually at Agaric we are generous, trusting souls, and we may want to update the project budget when we sent the retainer invoice, not when it is paid.