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
We're going to want to filter on this. What's the impact of changing this later? If it matters, we have an easy way to republish all documents to get them reindexed.