Ensure path ending with game ID always has a trailing slash

The trailing slash is essential here so that when JavaScript code
executing in the user agent accesses a relative resource, it is a
child of the game ID rather than a peer.

That is, from https://lmno.games/empires/WLWV/ a reference to the
"players" resource is https://lmno.games/empires/WLVW/players
(But if we started with https://lmno.games/empires/WLWV we would
end up referencing https://lmno.games/empires/players which will
not work.)

Here, we were already doing a redirect in the case of needing to
canonize the game ID, so we simply need to also do that redirect even
if the game ID was already canonical but the trailing slash was
missing.