Finish the actor upgrade implementation (plus invuln crates, EMP, and elite self-healing)

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.

Fixes #2599.
Not that this is ideal, but code could be written to allow for this sort of structure in the yaml, and trim unneeded spaces in lines before using them. Trim the Description, and then use a reader to trim each individual line. Allows for some readability in the yaml.