Published flag for documents

[epatters]

In addition to its permissions/access control list, a document should have a published flag stating whether it is publicly listed. Once a document is published, it cannot be unpublished, and a published document can depend only on other published documents.

This requires:

• adding a boolean column published to the document ref table
• enforcing the constraints above in the backend
• adding a toggle for publishing to the UI for permissions (Interface to set permissions on documents #276)

For further discussion, see Sec 3.1.2 of the n-dim report.