Grant Temporary control of Fleets to other Empires

**What do you want to see?**
Give Empires the ability to hand off control of fleets to their allies. While the fleets remain owned by them, the other empires can give orders and control them just like their own.

**How would it help players?**
Allies would be able to better coordinate their defense, while maintaining the ability to withdraw loaned fleets if needed.

**Alternatives**

**Things to consider**
Fleets will be ordered by Empires other than their owner, which means the usual checks will need to be changed. The empire loaned to should not be able to destroy the fleets outside of combat (throwing the loaned fleet into a battle it won't win should be okay though). Fighter and unit launches need to be maintained as the same owner.