Add a Phriction hint when a draft exists, and fix some draft editing bugs

Summary:
Depends on D19662. Ref T13077. See PHI840.

• If you're looking at the published version of a document, but a draft version exists and you can edit it, add a hint/link.
• Fix an issue where the "draft" transaction would complain when you created a document since the initial content is empty and no "draft" transaction is adding any content.

Test Plan: Created new documents, viewed documents with current published versions and unpublished drafts.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13077

Differential Revision: https://secure.phabricator.com/D19663