Portals are now actually portals

When the doors are open, the portals are active and can be walked
through. Added utility function Exterior:DoorOpen. Added entity hook
ToggleDoor. Moved door code into sh_doors. Set interior owner. E now
toggles door, Alt-E teleports in. Slightly adjusted portal positions and
sizes. Portal only renders when door is open, greatly improving
performance. Prevented physgun being able to pick up the interior. Added
spawnicon. Added addon.json for workshop.