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 really do anything).

However, I suspect that this behavior is almost always reasonable/correct, and that we can probably get away with it until this grey area between "notifications" and "external service calls" is more clearly defined.

Test Plan:

• Created a build with HTTP, CircleCI, and Buildkite steps.
• Put install in phabricator.silent mode: all three steps failed with "declining, because silent" messages.
• Put install back in normal mode: all three steps made HTTP requests.
• Read updated documentation.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13078

Differential Revision: https://secure.phabricator.com/D19085