add fight_or_flight_aliens.bt, use it

[sweet235]

This describes how alien bots decide when to fight and when to retreat and heal.

Reduce code duplication by moving this part of the alien bot behavior in its own file. Tweak it a little, please refer to the comments in the file for a detailed explanation. Summary:

• Abort healing when the base is under attack
• Do not make the decision to heal depend on the distance to the alien base

Edit:
Don't let the size of the file scare you, it is mostly comments.