R1:84020a363fee

Let Herald activation depend on which transactions are being applied, and generate transactions

Summary:
Ref T2222. Ref T4484. This is a stepping stone to getting Herald supported in the new Differental code. Generally:

- Instead of an Editor either supporting or not supporting Herald, let it choose based on transactions. Specifically, Differential only runs rules on revision creation and diff updates.
- Optionally, allow an Editor to return some transactions to apply instead of having to apply…