(mp_base) Skybox partitions (lobby, evac, 3D skybox etc.)

[snake-biscuits]

Scripts (gamemode, lobby) have some assumptions about where different parts of the map are
There's a common base shape to how mp_ maps are broken up

We should match how retail does it so those assumptions are still correct

Bonus

• light_environment_volume support would really help for Titanfall 2, since lobby lighting is consistent
• Some visual explainers of the layout would be a good resource for mappers