Remove duplicated IronCurtainable rules.
Add a UnitUpgradeManager trait.

This introduces support for timed upgrades, starting with crate buffs.
Fix cloak upgrade expiration.
Reimplement iron curtain as a generic stat upgrade.
Add the invulnerability crate to RA.
Add a GrantUpgradeWarhead and initial TS EMP implementation.
Replace RemoveOnConditions with KillsSelf.

Upgrades offer more flexibility than prerequisites.
Detonate demo trucks when they are iron-curtained.
Rename UnitUpgradeCrateAction to GrantUpgradeCrateAction.
Expose upgrades to map scripts.
Remove the now-redundant ScriptInvulnerable trait.
Merge pull request #6556 from pchote/upgrade-all-the-things

Add support for time-limited actor upgrades.