[Gameplay] Dog Kennel for Britons

Back by popular demand; patch requested by @borg-. This gives the "War Dog" Kennel building back to the Britons. Previously removed in Alpha 24. Its removal widely lamented since then.

War Dogs removed from the Briton Stable as a consequence.

Patch by: @wowgetoffyourcellphone
Accepted by: @borg-, @real_tabasco_sauce

Differential revision: https://code.wildfiregames.com/D5153