Add "phabricator.silent" for stopping all outbound events from an install

Summary:
Ref T7522. This is mostly useful in the cluster, but could be useful for external installs too.

If you want to import an instance into a test/dry-run state (in the cluster, to test an import; in the general case, to do something like test new hardware or configuration), you currently risk spamming users with a lot of duplicate notifications. In particular, if Phabricator tracks remotes, both instances will continue importing commits and sending email about them. Both instances will try to publish to mirrors, too, which could be bad news, and both instances will try to update linked services.

Instead, provide a flag to let an instance run in "silent mode", which disables all outbound messaging and data.

We need to remember to support this flag on any new outbound channels, but we add about one of those per year so I think that's reasonable.

Test Plan:

• Flipped config.
• Saw it void email, feed and mirroring.
• Didn't test SMS since it's not really in use yet and not convenient to test.
• (Can you think of any publishing I missed?)

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7522

Differential Revision: https://secure.phabricator.com/D12109