enable/disable system services and settings via API

[proddy]

Triggered by #2168 two new commands were added in 3.7.1 to control the shower (/api/system/showeralert and /api/system/showertimer).

A more generic approach is to extend the new action command, moving this from /rest/ to a HTTP POST API /api/system so we can use a format like {"action":'enable|disable', param:'ntp|ap|syslog|showeralert...'}.`

This will remove 3 EMS-ESP Commands and free up some memory.

[proddy]

Request to extend this to turn LED on and off as well

#2194