Configurable temperature data source path

Running Pi-hole on a $100 TUREWELL Mini PC (Debian) with the default temperature source configured in /var/www/html/admin/scripts/pi-hole/php/header.php returns 0C in the header as device temperature.

After manually changing the code to "thermal_zone3", i.e. to

    $output = rtrim(file_get_contents("/sys/class/thermal/thermal_zone3/temp"));

returns a more realistic temperature of 53C.

The temperature source path should be configurable. i.e. the user should be able to override the thermal zone or the whole string "/sys/class/thermal/thermal_zone0/temp" via some setting or a variable in /etc/pihole or some other persistent setting. This could be places alongside the units selector in the settings.php page.

As reference, the "sensors" outputs from the device was:

Adapter: Virtual device
temp1: +59.0°C

Adapter: ACPI interface
temp1: +0.0°C (crit = +100.0°C)

Adapter: ISA adapter
Core 0: +54.0°C (high = +90.0°C, crit = +90.0°C)
Core 1: +54.0°C (high = +90.0°C, crit = +90.0°C)
Core 2: +53.0°C (high = +90.0°C, crit = +90.0°C)
Core 3: +53.0°C (high = +90.0°C, crit = +90.0°C)

Adapter: Virtual device
temp1: +53.0°C