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.
It would be good for this scenario to test the hard case for previewing an existing draft:

1. Add an attachment to a draft
2. Save it
3. Add the attachment markdown into the body and preview (without saving)