Changes slug setter to put slugs on a solr document field named 'slug', ...

...instead of 'id', and leaves the 'id' field alone.

Removes the addition of a 'slug' datastream on Fedora objects. It's not being used.
So the old way was to use the slug as the solr id, and so a solr search by id was actually a search by slug... but it feels like what we need now is a `find_by_slug` method on `solr_connection`, and a corresponding test: We're not actually interested in searching by (Fedora) id, I don't think?