admin: Fix admin page to correctly show active/idle games and players

A while ago we changed the storage for players. Previously we had a
_players array and a separate clients array for their
connections. Then at some point in the past we changed to an array
named "players" and instead of separate "clients" each player in
"players" now has its own list of connections.

Ever since that change the admin view has been broken since it wasn't
updated to track that change. Here we bring it up to date, (including
the addition of two nunjucks filters, "active" and "idle" to help with
this).