Change "Shared resources" from vassal/liege to Trade Routes

Right now vassalization is extremely weak from an economic standpoint, with vassal tribute being very low and the only economic benefit being access to the vassals resources, which can be done more easily by taking the land or trading, and provides no additional benefit despite being more difficult. 

When another empire is vassalized, make it so that trade routes are automatically formed for every resource between the vassal and liege, so that each can profit from all trade route economy infrastructures and modifiers.