Refactor GraphDb.game_recommendations()

[mkazin]

1. It doesn't belong under GraphDb - a data structure. Move to gameRecommendations, probably as a static method
2. Also move the TraversalFilters
3. It is currently returning a hashmap of games. There's no real need for this, a list would be easier to understand. Instead, let's just have it returned the sorted list from (the now-misnamed) sort_by_playtime() (i.e. merge the two functions)
4. The method is already a little long, so let's break it up.