Fix incorrect assertions
Html pubs keep slugs after first published

Reworked the testing to include the changing of slugs whilst the
publication is in draft, and then to fix the slug after that with
successive publications.
Data migration: fix blank title/slugs in html pubs
Merge pull request #404 from alphagov/fix-blank-slugs-in-html-versions

Fix slugs not propagating correctly in HTML pubs

https://www.pivotaltracker.com/story/show/46557791