R1:ed7823f57716

Allow subscriptions to decline to generate an invoice

Summary: This is a useful capability in Phacility for disabled/suspended instances.

Test Plan: Used `bin/phortune invoice` to invoice a disabled instance, saw it decline to invoice.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11837