Add DB comparison indicators to the admin page

Addresses this [ticket](http://manage.dimagi.com/default.asp?121855#708110) (screen shot there too). These changes add a table to the admin page that displays a few comparisons between the number of docs in couch,  es, and postgres. This includes a modification to the couch users view I'm afraid.
@esoergel I guess I am not sure if all of the documents in the `cases` index are `CommCareCase`s. If there are sometimes other sorts of documents, then I think this filter is needed?