Feature: Sending a DELETE request to a uri optionally does not unpublish the page

[zonika]

Now, DELETEs to the _uris route kick off a page unpublish event. This is great for most use cases, except when multiple uris exist for one page. For example, we create a URI for the amp url of an article when the article is published, and we need to delete it when the article is unpublished. Now, sending a delete request to the amp url's uri kicks off an unpublish event even though this request is only really sent after the page has already been unpublished. Ideally there would be a query param available for delete requests that allows one to just delete the uri from the db without setting off the entire unpublish chain.