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This will enable us to remove unpublished manuals, freeing up their slug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).
I think we should be more explicit with this name that it's only for deleting draft manuals – `delete_draft_manual`