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. 
This should be `admin/categories`?

We could probably use the `resources` helper to generate the routes with the expected paths and path helpers (i.e. `admin_categories_path` rather than `admin_category_index_path`)