Sanity check player numbers

[Mailaender]

http://activity.openra.net/ can be easily junked with invalid data such as games with unrealistic large player numbers, which is what @martinalderson did when he tested his exploits OpenRA/OpenRA#11826. Similar to #5, but more annoying as our graphs then scale to the super high player count.

[pchote]

The new advertising protocol calculates these numbers based on the given client list. It doesn't solve the problem, but it does make less trivial to spoof.