Rename "Registrations" to "Registration"

[Trello
ticket](https://trello.com/c/e3yKYxH3/377-change-registrations-to-regist
ration-on-finder-frontend-for-aaib-reports)

I have also added an integration test for AAIB Reports, as that was
missing.
Merge pull request #56 from alphagov/rename-registrations-to-registration

Rename "Registrations" to "Registration"