Allow to limit unit count per match.

This allows to limit the number of times a specific template can be constructed/trained/created during a match.

Part of: https://wildfiregames.com/forum/index.php?/topic/27214-borg-expansion-pack-mod-implementation-in-0ad-alpha-24-release/
Refs. https://wildfiregames.com/forum/topic/24682-champions-and-civilisations-balance-mod-for-a23/

Differential revision: D2411
Reviewed by: @wraitii
Comments by: @Angen, @Stan