Support production of actors without IFacing.
Introduce IUpgradable and purchasable unit upgrades.
Reimplement UnitUpgradeCrate using IUpgradable.
Reimplement unit levels using IUpgradable.
Add FriendlyFire property to GivesExperience.
Replace CloakCrateAction with upgrades.
Merge pull request #6110 from pchote/unit-upgrades

Add an actor upgrade system (+ overhauls veterancy and crates)