Fix slugs not propagating correctly in HTML pubs

We discovered that HTML pub slugs were lagging one behind the edition,
and additionally that we want slugs to change in draft mode but not once
the HTML version is published. This PR fixes that by changing the logic
in `HtmlVersion` to regenerate slugs only if the edition hasn't yet been
published.

Also includes a data migration to fix the titles and slugs for old HTML
pubs that were allowed to have blank titles.

More information on the ticket:
https://www.pivotaltracker.com/story/show/46557791