3091 Have Helis take off after repair/rearm

As per polish request:
https://github.com/OpenRA/OpenRA/issues/3091

Will run the Helicopter Activity TakeOff() whenever a helicopter is fully rearmed or repaired, or constructed.
TakeOff() attempts to find the rally point for the building below it. If it does not exist, it will simply hover over the building. If it can't, it will simply hover over its current position.