Multi-square doors, cube vans, and other random vehicle stuff

The commit messages just about cover it, but here's a summary:

* Armored car now has a blueprint. (The old one has unescaped quote marks in it, which caused problems)
* Removed a reference to the now-deleted vpart_id enum from the documentation.
* Multi-square doors! They open all doors of the same type that are connected to them. To showcase this, the doors on the backs of cars have been switched out for 'trunk doors', and the doors on the backs of semis have been switched for 'shutter doors'. They use a generic MULTISQUARE flag that I hope can be used more in the future for stuff like caterpillar tracks.
* Cube vans! The delivery van kind. They have a shutter door on the back. Unfortunately, due to some craziness with wheels and boards not mountable together, you can see inside them even with the shutter down, but that's a separate issue entirely. They occasionally spawn in roads (replacing a few car chassis), in parking lots, and outside offices.