In general, `./scripts/restart-server` will already work in any
circumstance where the server is already stopped and needs to be
started. However, it will output a couple minor warnings, and it is
not readily obvious that it *will* work correctly.
Add an alias for `restart-server` named `start-server`, for
parallelism with `stop-server`, which omits the steps of
`restart-server` which would stop the server first.
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.
Merges the "Upgrades" section from production/maintain-secure-upgrade.md
with production/modifying-zulip.md.
Contains significant textual changes by tabbott to read more clearly.