FIX: Calculate no ads for groups server side (#200)

If the selected group to not display ads to had its visibility set to
not be visible then this setting wouldn't work correctly because that
group wouldn't be available client side. The change moves that group
check to be server side so that we can correctly see all the groups that
should not see ads.

lib/adplugin/guardian_extensions.rb

@@ -21,5 +21,9 @@ module ::AdPlugin
     def show_adbutler_ads?
       !self.user.in_any_groups?(SiteSetting.adbutler_exclude_groups_map)
     end
+
+    def show_to_groups?
+      !self.user.in_any_groups?(SiteSetting.no_ads_for_groups_map)
+    end
   end
 end