R1:6a4d5ce3c93e

Don't allow Herald Diff rules to "Call Webhooks"

Summary: Like "Commit Hook" rules, these also fire oddly and don't have an object PHID or a list of transactions.

Test Plan: Verified that "Call Webhooks" was no longer available from Diff rules, but still available from other rule types.

Differential Revision: https://secure.phabricator.com/D19069