API extension: mark player as defeated/left for a game, all others are marked alive/playing

[CrsiX]

In the response provided via #44, the defeated/left players should be included, though.

Only the player itself should be able to leave a running game (= get marked as defeated) for itself. At game start, nobody is marked. Whether the in-game 1:1-chat is retained after a player left the game is up to discussion. This endpoint is somewhat idempotent: resigning is only possible once and irreversible.

This may open the door for removing a game:
If every player is marked as left, the game can be cleaned up properly.

This functionality currently does not need to inspect game data. The client will know when it has lost and offer the player the feature to "delete" the game (only locally, of course), which will use that endpoint. Also, if a player wants to resign from a game, that endpoint will be used.
If only one player is left, it's either single-player or the victorious user: that user may quit any time using that endpoint as well, but the game will not do it automatically.