Refactor GraphDb's use of "center" #23
Description
GraphDb's use of NetworkX isn't fully implemented.
- insert_players() is only used by unit tests. Otherwise the graph is unpopulated
- As of now, game_recommendations() still performs manual BFS traversal using the provided "center" node
- Instead, we should investigate NetworkX's capability to traverse for us
- Instead of passing in the center node, the node's ID (steam_id of a player) should be provided, which should then be queried in the vertices.