Show the user when a node or DR host was last seen


In this example, the DR host and an-a01n02 havn't been seen in a while.

On the back-end; Create ```Database->get_last_seen``` and pull the most recent ```modified_date``` for each ```host_uuid```.

On the front-end, if the last update is over 5 minutes, grey out the host or otherwise show that it's stale. The time the system was last seen should be shown somewhere (ie: "Last seen: 30s ago" or something like that).