Pre-publication validations

This adds pre-publication validations and specifically a property that can be used by both authoring and webview to stop and/or disable publication. This boolean property is ``is_publishable`` in authoring and is sent to webview as ``isPublishable``.

This adds the following:
- an ``isPublishable`` boolean on binders and documents.
- an ``areContainedPublishable`` trivalue on binders, which indicates whether all, some or none of the binder is publishable.
- ``isPublishable`` added items in a binder's tree. This property is a boolean that is only added to the binder itself and draft items in the tree.

See also the [trello card](https://trello.com/c/nxo0aMkb).