ADS-B station: A small improvement that pleases me

ADS-B station prominently shows the number of aircraft in its range. To spruce up this tiny feature, I've tweaked it so that the background color of the counter badge changes depending on the current number.

• Yellow: the number of aircraft is lower or equals the specified minimum (default is 29)
• Green: the number of aircraft is higher or equals the specified average (default is 30)
• Dark lilac: the number of aircraft is higher or equals the specified maximum (default is 70)

The minimum, average, and maximum values can be specified by editing the $AIRCRAFT_COUNT array in the config.php file.

Although it's not a particularly useful feature, it pleases me to watch the count badge changing colors.