Issues/222 category editing add order interface

Builds on https://github.com/mysociety/alaveteli/pull/1721 to add an admin interface for reordering headings and categories. 
The `private` keyword has no effect on singleton methods: https://eval.in/196882.

You really want [`private_class_method`](http://www.ruby-doc.org/core-1.9.3/Module.html#method-i-private_class_method)