webadmin: Implement sorting by simple status image

Implemented a couple of helper classes to sort simple status
(i.e. none, down or up) columns in a generic way, and applied this
logic to the Network/Clusters, Network/Hosts and Network/VMs subtabs.

Change-Id: I9ac043ef1d522f67678b5c70b461833a82d3d9f0
Signed-off-by: Lior Vernia <lvernia@redhat.com>