Load list of slugs lazily

[tobinus]

Even if the list of slugs isn't used, it is currently loaded from the database every time an instance of SlugList is made. Finding the slugs requires a full scan of the table slug_to_slug, which is wasteful if it's not going to be used.

A better approach would be to only fetch the list of slugs once requested.

• Store slugs from constructor in self._slugs
• Use self._slugs in canonical_slug (so it won't trigger fetching the list of slugs)
• Fetch and return list of slugs from the database when self.slugs is referenced.
• Do so only the first time self.slugs is referenced, and populate self._slugs. On subsequent references, return self._slugs.
• Don't fetch list of slugs in SlugList.from_slug().