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.
If this becomes common (importing multiple `ESQuery` classes into the same file), we might want to import them to `corehq.apps.es.__init__` so you can do `from corehq.apps.es import CaseES, DomainES, FormES`