Fix server freeze on client absence.
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.
Mods.Common Widgets

Moved over Widgets that don't require a bunch of things to move.
Extended IMove for Enter logic
Extend Enter activity class for (re)usability in enter/exit/dock logic
Add support for generic tooltip names.
Add classic-style tooltips for TD campaign missions.
Fix bogus fence tooltip.
Add classic-style tooltips for RA campaign missions.
Merge pull request #6556 from pchote/upgrade-all-the-things

Add support for time-limited actor upgrades.