Attachments can be updated

* Titles may be changed
* Files may be replaced
* Does not handle cleaning up old assets
* Does not handle or warn if out dated markdown snippets are left in body
Update govuk_content_models to 10.4.0

In order to use Attachment update behaviour.
Merge pull request #72 from alphagov/replace-attachements

Attachments can be updated
Actually test that publication happens
Extract some services
More service layer refactoring
Name parameters in SpecialistDocumentBuilder
Acceptance test for unique slugs
Validate uniqueness of document slugs
All Panopticon conerns moved into service listeners
Fix document seeding
Compromise separation of concerns for compatibility