A new "terminal" plot type for T gate integration.

[Pheotis]

Towns and nations should be able to make their own nexus systems.
This would use the new Terminal system, and would be closely related to a town/nation's portal network.

Perhaps %nation_name%_terminal or something like that.

Towns and nations should have a custom plot type called "Nexus".
Town and nation portalsmiths (see #2) should, by default, be able to work with such portals.