Landing page for topic detail view is now concept graph

When you click on topic, the landing page for topic details view is the topic's concept graph.
Also removed the bug while saving the topic node contents.

Also by taking the suggession from Avadoot regarding the error as "Node has no attribute ..." for node fields ,
Intersection between current node object keys and rcs nodes object keys would gives us the node which have the keys which are not present in rcs version of that node, for which we need to set the value as none. 
So removed bug for showing this intersection, in such a way that , it should compare the current node keys and rcs version of node keys rather than comparing with node class fields.
modification in : 
 ```models.py``` --> included the current node keys as "document_object.keys()  in get_version_document()"