Add support for fetching locations by their slugs

This allows web clients to maximize their SEO by using human readable URLs as opposed to numerical IDs to represent locations.
Use separate slugs for Organizations and Locations

If Location belongs to Organization, and a location's slug is composed of both the location name and the organization name, mongoid-slug can't keep track of changes to an organization's name, so a new slug won't get generated. This was the wrong approach. It's best to provide a separate slug for each and let web clients use that to construct their own URL paths.