[ENHANCEMENT] - Allow easy changing of ALLSKY_IMAGES location

[EricClaeys]

Many people have an SSD attached to their Pi and they want to use it to store images.

A proposed solution:

Add a "Change location of saved images" menu item. It will:

• Prompt the user for AN EXISTING location.
• Prompt the user if they want to move all existing images to the new location.
• Create uservariables.sh if not already there. Either way, set ALLSKY_IMAGES to the new location.
• Stop Allsky
• Update /etc/lighttpd/lighttpd.conf and ~/allsky/html/includes/allskyDefines.inc with the new location.
• Optionally move existing images to new location.
• Start Allsky