Implement logging of the lobby

If `loglobby` is enabled in the config file, lobby logs are written
to logs/lobby/yyyy-mm/yyyy-mm-dd.txt. Lobby logs are kept in their own
subdirectory of the 'logs' directory in order to avoid a namespace
clash with a format named 'logs', and also because there are many
reasons why one might want to copy the battle logs without copying
the lobby logs.