Adding state to document series so that we can soft delete

Extracted code out of classification, to make a SimpleWorkflow module.
SimpleWorkflow tests

Consolidate tests, and specify the initial state.

Also skip index removeal if searchable is not defined
Delete button

Needs styling
Controller test for document series delete
Explicitly state we should overwrite the destroyable? method
Merge pull request #355 from alphagov/delete-document-series

SimpleWorkflow: Deleting a document series 

https://www.pivotaltracker.com/story/show/46108789