Bug 893761: Improve revision dashboard performance.

This reorganizes the view code a bit to gather all the query kwargs in
one place, and adds a couple optimizations:

1. If no locale, username, topic or new-user filters are being
   applied, does a straight count() on the revisions table to get the
   number of Revisions. This should be a bit of a saving.

2. If the starting number for the current page is less than the total
   number of Revisions we'll get, skip querying for the Revisions at
   all. It's a waste of a DB query, since we already know from the
   total that it's a waste of a (complex) DB query.