R1:bd2eaad04f03

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…