Display pagination links on the latest changes view

This changes the latest changes view to include links to the previous
and next pages of changes, where available.

As much of this behaviour is already encapsulated in the 
ChangedDocumentsPaginationPresenter class, this adds a helper method to 
the SubcategoriesController which builds a presenter instance.

This adds a 'helper' stylesheet for pagination links, which has been
copied from the existing one in Frontend [1]. It also adds an 
integration test for moving between pages of the latest changes feed.

[1]: https://github.com/alphagov/frontend/blob/c42fb2bb/app/assets/stylesheets/helpers/_pagination.scss