pagination: avoid counting pages outside searches.

Replace this common pattern in controllers:

    @tags = Tag.search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])

with this:

    @tags = Tag.paginated_search(params)

`search_count` is used to skip doing a full page count when we're not
doing a search (on the assumption that the number of results will be
high when not constrained by a search). We didn't do this consistently
though. Refactor to do this in every controller.

app/controllers/post_versions_controller.rb

@@ -6,7 +6,7 @@ class PostVersionsController < ApplicationController
   respond_to :js, only: [:undo]
 
   def index
-    @post_versions = PostArchive.includes(:updater, post: [:versions]).search(search_params).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @post_versions = PostArchive.includes(:updater, post: [:versions]).paginated_search(params)
     respond_with(@post_versions)
   end