SimpleWorkflow: Deleting a document series 

Extracted a `SimpleWorkflow` module for delete flags, due to limitations with the statemachine we have, this cannot be used in-conjunction with `Edition::Workflow` so don't include it into any subclass of `Edition`.

We also expect a `destroyable?` method to be defined as the guard method for checking if it is removed from all associations.

This is for deleting document series: https://www.pivotaltracker.com/story/show/46108789