Allow deletion of a draft manual via a bin script

This will enable us to remove unpublished manuals, freeing up their slug
for use by other manuals (perhaps which have been made in the wrong
organisation, for example)

This will enable the second part of [this trello ticket](https://trello.com/c/gc49XBvG/342-chore-content-design-manual-change-slug-and-remove-old-content).
The slug is easier for us to obtain than the manual_id, and easier for a content team to provide to us -- log in to specialist-publisher, copy the grey text under the manual, send it to us, "I created this by mistake, can we delete this?" as has happened in the case of the content-design manual.

I guess we *could* ask them to copy and paste the entire URL and pull the manual id from that, but it seemed like a nicer interface.