When "phabricator.silent" is enabled, don't call webhooks

Summary: Ref T13078. The phabricator.silent configuration flag should disable webhook calls, since this is consistent with the documented and desired behavior.

Test Plan: Enabled phabricator.silent, made test hook calls, saw them fail with a "silent" failure reason.

Maniphest Tasks: T13078

Differential Revision: https://secure.phabricator.com/D19084