Rake task to delete a slug from search

As part of changing a mainstream slug we need to first delete the slug from panopticon. This rake task will help to make that process easier.

See:
https://github.com/alphagov/wiki/wiki/Changing-GOV.UK-URLs#making-the-change
This needs a leading `/`.  We should also make it fail noisily if the slug isn't given, or is blank.