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/forum_posts_controller.rb

@@ -24,8 +24,7 @@ class ForumPostsController < ApplicationController
   end
 
   def index
-    @query = ForumPost.search(search_params)
-    @forum_posts = @query.includes(:topic).paginate(params[:page], :limit => params[:limit], :search_count => params[:search])
+    @forum_posts = ForumPost.paginated_search(params).includes(:topic)
     respond_with(@forum_posts)
   end