Allow searching Specialist Documents by Slug and Title

Some of our document formats (AAIB and Drug Safety Updates, for example) have many documents, and our publishing tool doesn't easily allow users to find a document to amend other than by paging through the
results.

This change allows users to search by either slug or title using a form on the list page.

I have made the decision to truncate the results if there are more than the Kaminari pagination limit, and show a warning to the user. The alternative to this is to write another `Paginator` or modify the existing one to gracefully handle the searching case, which was a level of complexity I didn't feel was required for this task.

This behaviour was run past Holly.

I'm not entirely happy about the tight ties the tests have with AAIB reports, but as they're a subset of specialist documents I don't think it really matters.

**Reviewing**
Please review commit by commit.