Look into better doc list partial override solution

[mejackreed]

The current solution of the mustache like template is limited, and actually requires the document list to be processed twice (once server side, once client side).

Look into modifying json response to process documents individually through the server side template rendering: https://github.com/sul-dlss/blacklight_heatmaps/blob/master/app/views/catalog/index.json.jbuilder#L6