Manual documents

Functionality to add documents (with attachments) to manuals.

You may notice the lovely hack to get around CMA Case specific validations on documents that are currently in the content models gem and also there is some serious work to do refactoring the cucumber steps when we've decided how to separate different types of docs.

EDIT: There's also an awful lot of controller and view duplications that we'll need to refactor soon.

https://trello.com/c/CIVqY1uh/63-manuals-add-document-to-a-manual-5