Allow searching Specialist Documents by Slug and Title

[Trello ticket](https://trello.com/c/SRgfeSGA/360-searching-for-documents-in-specialist-publisher-5)

![Screenshot](https://dl.dropboxusercontent.com/u/39836361/Screenshot%202014-10-22%2013.31.47.png)

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.