Fix for ManualDocuments Preview not working

We were missing an action which wasn't needed until some previous refactoring changed how the Preview worked. I've added some tests to make sure this doesn't happen again along with adding a service for previewing manual documents.
You could extract a `manual` method here, and/or wrap the line a bit:

```ruby
@existing_document ||= manual.documents.find { |document|
  document.id == document_id
}
```