Local resource mode

**What do you want to see?**
Inspired by Stars!, this would be an option in game setup which causes resources such as minerals to be local to a planet and require transporting around by freighters to build stuff. Space ports (and Natural Merchants) would automatically distribute resources as needed in a system, but you would need a transport ship to get them to your interstellar colonies.

**How would it help players?**
More things for grognards to micromanage! Cut off enemy supply lines!

**Alternatives**
N/A

**Screenshots and mockups**
N/A

**Things to consider**
* Would spaceports transport resources arbitrarily between colonies in a system, or only pull them in from other planets, or only push them out?
* Would there be a limit on how many resources can be moved per turn?
* Would you need to send your freighter to where the resources physically are to get them, or could you load them from any colony in the system provided you have a spaceport? Or maybe manual transfers using spaceports?