[feat] Watchtower Healthchecks

[kn0wmad]

Optional healthcheck (if a watchtower client has been added) to let user know if (and perhaps how many) clients are online.

May also be desirable to have a watchtower server healthcheck to verify that your server can be reached by remote clients.

This may start to crowd the service page. Perhaps this real estate is being reconsidered for 036/040

[Dominion5254]

#152 introduces a conditional health check for the watchtower server (if enabled).

Will explore a conditional health check for the watchtower client in the somewhat near future as well.