Add a `bin/config set <key> --stdin < value.json` flag to make CLI configuration of complex values easier
Summary:
Depends on D19003. Ref T12677. Ref T13053. For the first time, we're requiring CLI configuration of a complex value (not just a string, integer, bool, etc) to do something fairly standard (send mail).
Users sometimes have very reasonable difficulty figuring out how to `./bin/config set key <some big JSON mess>`. Provide an easy way to handle this and make sure it gets appropriate callouts in the documentation.
(Also, hide the `cluster.mailers` value rather than just locking it, since it may have API keys or SMTP passwords.)
Test Plan: Read documentation, used old and new flags to set configuration.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T13053, T12677
Differential Revision: https://secure.phabricator.com/D19004