Allow updating date/time via web console

[ingojaeckel]

Accurate date/time on the Pi is important as it is used in file names each time a photo is taken. With a battery backed RTC present, time should only need to be set once. For RTC-less Pi deployments adding this feature may be more useful. In those deployments, the Pi's time may restart at the same point on each reboot. This could lead it to overriding files created in a previous run as the Pi would have no concept of 'earlier' without the RTC.