R1:4fa99374be1c

Prevent "Call webhooks" Herald action from appearing in UI for adapters which can't fire it

Summary:
See <https://discourse.phabricator-community.org/t/herald-webhook-is-not-called-but-herald-transcript-tells-me-the-opposite/1098>.

The "Commit Hook" events don't operate on objects and don't use TransactionEditors. They can't call webhooks in a normal way and currently don't call webhooks at all. Stop offering these actions in the UI.

The "Outbound Mail" event also fires oddly and likely doesn't make…