Add a controller for each document type

Kaminari, internally, uses url_for(controller:, action:) to generate
routes. See
https://github.com/amatsuda/kaminari/blob/adca9b25b4d3fefcb12552e939d726
2eb86a3ec0/lib/kaminari/helpers/tags.rb#L30

In specialist-publisher, we had 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 were linking to /aaib-reports?page=2.

To fix this, we have created a controller for each document type that
simply inherits from AbsctractDocumentController. While creating
additional code, this brings us closer to standard rails routing. We
have decided to prefer this to potentially confusing hacks around this
problem.
Merge pull request #303 from alphagov/controller-per-document-type

Add a controller for each document type