Supplying migrations for a bold new era of mediagoblin entry slugs

Supplying migrations for a bold new era of mediagoblin entry slugs

Okay, that's a totally confusing statement, but the docstring of this
migration summarizes it well:

  Entries without slugs now display differently in the url like:
    /u/cwebber/m/id=251/

  ... because of this, we should back-convert:
   - entries without slugs should be converted to use the id, if possible, to
     make old urls still work
   - slugs with = (or also : which is now also not allowed) to have those
     stripped out (small possibility of breakage here sadly)

This commit sponsored by John Sullivan.  Thanks johnsu01! :)