HDD space threshold and warning

[femto-code]

Discussed in #31

^{Originally posted by wannadrunk September 4, 2021}
Just realize there is no disk space monitoring. I have a bash script with cron job to monitor and send the notification to LINE Notify.

bash script - https://gist.github.com/wannadrunk/05b011cc1a80d6b5559e7c3b0532f27a

It would be also great and useful to add this disk space monitoring to RPi Dashboard. Maybe have the same graph as Memory.