Feature request: Upgrade units inside armies

[dobragab]

One of the important strategic decisions in the game is whether to fill armies with soon-to-be obsolete units, or keep the empty armies for later to be more effective.

For this reason, I tend to not fill armies with Ancient Cavalry, Knight or Tank, wait for Cavalry or Modern Armor. Which makes my attacks way less effective.

A trivial solution for this would be to create an Upgrade feature for Army, which upgrades all upgradeable units inside at once, and makes the Army skip the turn.