d3 tree implementation for theme map

* ```theme.html```  --> two tabs displayed on top of theme page named as "Tree Browser" & "Collapsible Tree"

* *Tree Browser*  --> shows previous theme map. 
   *Collapsible Tree* --> shows d3 collapsible tree for theme map.

* The reason for keeping both options to display theme map is some browsers may or may not be supportable to display d3 tree.  

* Regular tree fold & unfold options are placed under dropdown on hover of "Tree Browser" tab. Currently, for regular tree only there is fold & unfold options , later on will include it for collapsible tree as well.

* ```ajax_views.py```  --> modified ```get_tree_hierarchy()``` method  used to render json for both 'Tree Browser' & 'Collapsible Tree'