Don't assume document at slug exists

This prevents exceptions being raised when an attempt is made to preview
an HTML attachment on a document that does not exist.
Prevent preview of access-limited HTMLAttachments

This correctly scopes HTMLAttachment preview to the loaded edition and
also enforces permissions check for access-limited publications.
Don't raise when we can render
Merge pull request #1520 from alphagov/handle-html-attachment-preview-on-non-existent-docs

Handle html attachment preview on non existent docs