Add facility for object specific behaviors

Adds `SolrDocument#display_type` which creates a partial name string to be used in the overridden `#document_partial_name` helper method.

No longer overriding `_index_default` and `_show_default` partials and letting Blacklight handle them for fallback behavior beyond what we've identified (i.e. just printing solr field values).

This PR only implements marc behavior (i.e. `_index_marc` and `_show_marc`) but does also add a fallback `_record_metadata_default` partial that will render the area beneath the upper metadata section on the record view.  The `_record_metadata_default` partial only renders the access panels section (since some of them can be displayed backed 100% by index data) but we can add more later.