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- Published cases can be removed from the live site and set in a 'withdrawn' state that is functionally similar to being in draft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

What this looks like:

![screen shot 2014-04-30 at 09 36 08](https://cloud.githubusercontent.com/assets/74812/2839183/83516730-d042-11e3-8e2f-c49a4770f098.png)

Clicking on withdraw gets you this:

![screen shot 2014-04-30 at 09 36 52](https://cloud.githubusercontent.com/assets/74812/2839186/96e8a4ac-d042-11e3-89d7-d24eee392ff9.png)

Clicking the withdraw changes the status and redirects back to the index page.

New drafts can be created for withdrawn, but the status reflects this by showing "withdrawn with a new draft". Similarly, published documents with a new draft can be withdrawn and will also become "withdrawn with a new draft"