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.
the goal of the test here is to check the setting of the slugs, not the looking up by slug. The new `Openvault::SolrHelper` module handles that, but I forgot to commit a test in the PR. Sorry bout that, and thanks for the extra eyes to help catch it. Just submitted a new PR.