Essential Settlers

This new system adds an immersive way to make your Settlers essential (not killable by even you)
 * By having at least 1 doctor in the settlement, you can toggle the option (per settlement)
 * This option can be found in the "Local Settlement Stats" menu, from the Command Table or Recruitment Station
 * This option is OFF by default, even if you qualify for it, so you need to turn it on
 * This option is not global, and only works PER settlement
 * If a doctor is no longer available in a settlement, the option will eventually turn off at certain times
 * (in the future I may allow a global option to automatically turn it on based on the doctors, but for now I'd like to keep the system like this first)