Add disable_popularity debug parameter

Adding debug=disable_popularity disables the use of popularity
information when searching.  This is useful for checking the effect of
popularity information on searches.  This can be combined with the
existing disable_best_bets by joining the options with a comma
separator.
I've changed it to use assert_no_match, but I think we do need to make it into a string, because `query` is a hash containing deeply nested objects, and I'm just wanting to ensure that the string "popularity" doesn't appear anywhere in the tree.