Finish the actor upgrade implementation (plus invuln crates and EMP)

This adds support for limited-time upgrades and upgrades triggered by support powers and warheads.  It builds on the initial implementation from #6110 to make it more useful for modders and more worthy of a mention in the upcoming playtest news post.

These changes are demonstrated by porting the Iron Curtain to use upgrades, adding an invulnerability crate to RA, and implementing the plumbing for EMP in TS (but doesn't attempt to fix the other limitations).  This should also allow @Phrohdoh to finish his stealth generator implementation.