Hg remove empty ps

This fixed the problem of <b> tags wrapping <p> tags and creating invalid HTML when pasting from Google Docs. 
Does this work? If so that's interesting because we're not giving the editor focus before inserting the HTML: https://github.com/guardian/scribe-test-harness/blob/e5064f0a8dfec97ffd9c6916e5609c229fccce46/helpers.js