A revised algorithm for generating slugs.

A revised algorithm for generating slugs.

This one does not *force* slugs, but usually it will probably result
in a niceish one.

The end *result* of the algorithm will (presumably, I have not tested
it) result in these resolutions for these situations:
 - If we have a slug, make sure it's clean and sanitized, and if it's
   unique, we'll use that.
 - If we have a title, slugify it, and if it's unique, we'll use that.
 - If we can't get any sort of thing that looks like it'll be a useful
   slug out of a title or an existing slug, bail, and don't set the
   slug at all.  Don't try to create something just because.  Make
   sure we have a reasonable basis for a slug first.
 - If we have a reasonable basis for a slug (either based on existing
   slug or slugified title) but it's not unique, first try appending
   the entry's id, if that exists
 - If that doesn't result in something unique, tack on some randomly
   generated bits until it's unique.  That'll be a little bit of junk,
   but at least it has the *basis* of a nice slug!