zulip: Start monitoring it

zulip: Start monitoring it

We don't have much in the way of service-specific checks, but at least we can
make sure it's doing HTTPS and check for rate-limiting since that has come up.

Upstream Zulip has a bunch of stuff --
https://zulip.readthedocs.io/en/latest/production/troubleshooting.html#monitoring
-- that I should look into borrowing from eventually.