Turn document to draft automatically when updating it and remove "Edit draft" button

Currently, the user has to hit the "Edit draft" button to turn a document to draft. This issue proposes to replace this action by a listener which updates a document to draft automatically when PublicationWorkflow#isModified(originalDocument, updatedDocument) returns "true", and by a hint in the workflow area documenting this behaviour. Motivation: make the transition to draft simpler.