Withdraw document

CMA need to unpublish a number of cases that were accidentally created in duplicate. This will be an ongoing need that we must be able to deal with swiftly.

Acceptance criteria:
- Published cases can be removed from the live site and set in a 'withdrawn' state that is functionally similar to being in draft
- Using the old URL will return nothing
- This story does not include a user interface for editors to do this work

What this looks like:

![screen shot 2014-04-29 at 17 46 53](https://cloud.githubusercontent.com/assets/449004/2831826/e5a7a870-cfbd-11e3-9021-8610769f416e.png)

Clicking the withdraw reloads the page and changes the status. New drafts can be created, but the status reflects this by showing "withdrawn, with a new draft".