Fix exception when visiting accession in public app

To replicate:

1) in the staff interface create an accession without a title (so only with a four-part-id).
2) check the publish checkbox so it is delivered to the public app
3) in the public app, search and navigate to that accession
4) throws an exception `undefined method `length' for nil:NilClass`

Fixed by using the accession `display_string` rather than the `title` in the breadcrumb trail.