Create hasUserSeen() for tracking comparison

Would be lovely to provide a way to ask has this user seen something. This will invariably we used for different types of things: taxonomy, nodes, urls, so it should accept a type param.

// Easy.
hasUserSeen('node', nid)
hasUserSeen('term', tid)

//Maybe?
thing = { entity: { 'type': 'node', 'bundle': 'article' } };
hasUserSeen(thing, id)