Certbot replaces the cert files under /etc/letsencrypt/live/,
which our nginx config refers to symlinks to; but it doesn't
tell nginx there's been an update, so nginx keeps serving the
old cert.
This is fine as long as nginx is restarted, or just told to
reload its config, at some point before the cert actually
expires about 30 days later. Which is probably the common
case, but of course we should make it just work. So, if we
actually renew a cert, tell nginx to reload its config now.
This causes the cron job to run only when a Zulip-managed certbot
install is actually set up.
Inside `install`, zulip.conf doesn't yet exist when we run
setup-certbot, so we write the setting later. But we also give
setup-certbot the ability to write the setting itself, so that we
can recommend it in instructions for adopting certbot in an
existing Zulip installation.
Greg Price