Allow dashboard maps to be disabled

Add a configurable setting that allows the maps displayed in the farm dashboard, farm areas page, and farm asset list pages to be turned off.

This was a request made by a farm that is the Fijian islands, where the Google Map layers are not sufficient, so they prefer not to use the farmOS mapping features at all. Allowing the map to be turned off can greatly speed up the page load time, and reduce the overall amount of stuff on the page.

I see this as a good first step towards a future goal I have: create a collapsible map that is displayed throughout the farmOS UI, which changes based on the context of the page you're in, and can be hidden ("collapsed") to the side (saved in a cookie variable between page loads to remember your preference).