The previous setting caused the service to hit a rate-limit when it was
restarted more than 5 times in 24h.
Editing the Caddyfile and restarting the service could also easily
trigger this rate limit.
One could argue that users could simply call `systemctl reset-failed
caddy` to reset the rate-limit counter, but this is counterintuitive
because most users won't know this command and are possibly unaware that
they had hit a rate-limit.

The service is now allowed to restart 10 times in 10 seconds before
hitting a rate limit.
This should be conservative enough to rate limit quickly failing
services and to allow users to edit and test their caddy configuration.

This closes #1718

Remove restart limit settings and use defaults

By default 5 restarts within 10 seconds are allowed without
encountering a restart limit hit, see  `man systemd.unit` for details.

Set Restart to on-abnormal

The table in https://www.freedesktop.org/software/systemd/man/systemd.service.html#Restart=
shows the conditions for which on-abnormal would restart the service.
It will *not* restart the service in the following cases:

- a non-zero exit status, e.g. an invalid Caddyfile
- a zero exit code (or those specified in SuccessExitStatus=) and a clean signal
    clean signals are SIGHUP, SIGINT, SIGTERM or SIGPIPE
    https://github.com/systemd/systemd/blob/3536f49e8fa281539798a7bc5004d73302f39673/src/basic/exit-status.c#L205

The service *will be restarted* in the following cases:

- a unclean signal, e.g. SIGKILL
- on start and watchdog timeout (we don't use those systemd service
constructs explicitly)