Prevent scheduling of editions with bad links.

https://www.pivotaltracker.com/story/show/61197910

After deploy, this ticket requires us to review all scheduled documents to make sure they won't fail link validation when being published.

There's also more work to be done at some point in terms of splitting away the concept of "can be published" from the concept of "should be published" and the wording that comes along with that in the interface.