Using `supervisorctl stop all` to stop the server is not terribly
discoverable, and may stop services which are not part of Zulip
proper.
Add an explicit tool which only stops the relevant services. It also
more carefully controls the order in which services are stopped to
minimize lost requests, and maximally quiesce the server.
Locations which may be stopping _older_ versions of Zulip (without
this script) are left with using `supervisorctl stop all`.
Fixes#14959.
Alex Vandiver