Hack around kaminari's inability to handle generic controllers.

This is a TEMPORARY fix to allow pagination to work until we fix pagination properly.

Kaminari, internally, uses url_for(controller:, action:) to generate routes.
See
https://github.com/amatsuda/kaminari/blob/adca9b25b4d3fefcb12552e939d7262eb86a3ec0/lib/kaminari/helpers/tags.rb#L30

In specialist-publisher, we have a route for each report (aaib_reports_path)
which all use the same controller and action. When Kaminari calls
url_for(controller: "abstract_documents", action: "index"), the first matching
result in routes.rb is returned, which is AAIB reports. So the CMA cases
pagination links are linking to /aaib-reports?page=2

Override how page_url_for(page) works in specialist-publisher to instead take
an extra parameter of document_type and call the correct routing helper.
Merge pull request #301 from alphagov/fix-pagination-always-pointing-at-aaib-reports

Hack around kaminari's inability to handle generic controllers.