Migrate gameLength preference to LeagueProfile/GameClassificationLevel settings

The global `gameLength` setting is really game classification-dependent. As such, it's probably better if we add this as a field to the `GameClassificationLevel` instead of where it is now.

We should also add a default `gameLength` value to `LeagueProfile` objects.

Locate the current global `gameLength` setting and the definitions or entry points for `GameClassificationLevel` and `LeagueProfile`. Trace how the setting is read, then move its classification-dependent value and add a default for `LeagueProfile`; the work is done when both object types expose the intended settings without relying on the old global location.

Written by the indexing model from the issue text.