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. 
Maybe we could break this down a bit like:

```ruby
if category.is_a?(Array)
  add_from_array(category)
else
  add_from_params(category) # or whatever its adding from?
end
```

Not quite sure what the different cases are but I expect two named methods would help make the distinction