Import aaib reports from json files

([Trello ticket](https://trello.com/c/PMWRHmRc/187-aaib-finder-import-content))

From `import/metadata` (json files, see spec/fixtures/import) and `import/downloads`, import AAIB reports.

Also create an optional decorator used to validate that all attachments for a document have been referenced in the body of that document.

There is a requirement to allow summaries to be blank initially on import, but force them to be filled in as normal when an aaib report is edited manually, hence the wrapper in a hack that forces an extra save.

- [ ] TODO: Extract `validates :summary, presence: true` from SpecialistDocumentEdition.

Worked on by myself and @bestie
What's the difference between `registration_string` and `registrations`?

Can we alphabetise this list instead of adding the fields in seemingly random positions?