refactor list_controller

- minor refactoring of actions 'category' and 'category_feed'
- fix defect in 'category' where check was for literal
  string 'uncategorized' instead of SiteSetting.uncategorized_name
- major refactoring on defined topic actions

app/controllers/application_controller.rb

@@ -284,10 +284,18 @@ class ApplicationController < ActionController::Base
       render status: status, layout: 'no_js', formats: [:html], template: '/exceptions/not_found'
     end
 
-    protected
+  protected
 
     def api_key_valid?
       request["api_key"] && SiteSetting.api_key_valid?(request["api_key"])
     end
 
+    # returns an array of integers given a param key
+    # returns nil if key is not found
+    def param_to_integer_list(key, delimiter = ',')
+      if params[key]
+        params[key].split(delimiter).map(&:to_i)
+      end
+    end
+
 end