Added graph views,templates
Refs #13569
Added js template for graph_object. #TODO: views.graph.ClusterGraphView
Refs #13569
Shifted JS graph objects creation logic to views from templates
Refs #13569
Solved the "None" string problem, when there is no secondary node for an instance
Refs #13569
Removed some redundant imports in ganeti_web/views/graph.py
Refs #13569