R1:f74e6bbf8d70

Make "phabricator.silent" disable build steps which rely on external services

Summary:
Depends on D19084. Fixes T13078. When `phabricator.silent` is enabled, immediately fail the "HTTP Request", "CircleCI" and "Buildkite" build steps.

This doesn't feel quite as clean as most of the other behavior of `phabricator.silent`, since these calls are not exactly notifications in the same way that email is, and failing to make these calls means that builds run differently (whereas failing to deliver email doesn't…