Feature/displaying aaib reports

Allows us to display AaibReports on specialist-frontend. Also refactors common presenter code from the CmaCasePresenter into an Abstract DocumentPresenter.

Relies on new version of govuk_content_models gem being added to panopticon and specialist publisher. Currently awaiting a gem version bump but the merge request which adds the required changes to the models gem can be found here 

https://github.com/alphagov/govuk_content_models/pull/185