Add port of scg04ea mission

This is a pretty straight-forward port of scg04ea.  I tried to keep it pretty close to the original, but made it a little harder in places by adding some reinforcements that were disabled in the original and making some units less passive on normal skill.

It's still quite easy, though. You can run through it in less than a minute if you're fast, and three minutes is well over par, I'd say.

Needs #4854 to work.