Relate Snipe-IT Service to managed assets

[aiwilliams]

There is currently no relationship created between the asset/hardware and the Service. By creating a mapped relationship of Service - (MANAGES|TRACKS|INVENTORIES|GODHELPUS) -> hardware, one could find hardware that is managed by Snipe-IT in contrast to hardware that is not, thereby perhaps even satisfying some needs laid out in #11. This relationship is more easily visualized in the graph than having to look for a property on each entity returned by find hardware.