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 much sense to hook anyway.

Test Plan: Verified that these events no longer offer "Call webhooks", while normal events still do.

Differential Revision: https://secure.phabricator.com/D19061