added huge RA KotH map ATHENA

The map has been tested with four players and was reasonably lag free.
Note that this is the newer fixed after the multiplayer playtests - the
one on the content openra is outdated.

While the map is working perfectly fine, it would be great if
https://github.com/OpenRA/OpenRA/issues/3496 could be implemented. That
way, the player would have multiple starting options.