Implement `/master/pgn/{game_id}`#57

kraktus merged 5 commits intolichess-org:masterlichess-org/berserk:masterfrom

Anupya:Anupya/add-otb-master-game-endpointAnupya/lichess-berserk:Anupya/add-otb-master-game-endpointCopy head branch name to clipboard