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
I feel like this class would make more sense as a presenter-ish thing, which takes the raw data as a constructor and has an `attributes` or `to_hash` method that does all of the massaging.

I'd move the behaviour that does the repo checking and version destroying out to somewhere else.

That said, I don't see that a reason to hold up merging this, especially if it gets the job done.