Add AAIB Reports schema

In order to expose AAIB reports in search, we've added the schema for
the facets on the reports.

Trello ticket:
https://trello.com/c/X4vUOJzE/203-add-aaib-reports-into-search-results-2
Merge pull request #259 from alphagov/add-aaib-reports-schema

Add AAIB Reports schema