bug fix for browsing collection management records that belong to accessions without titles

A quick fix for a small bug I just bumped into.

Collection Management records can belong to Accessions. Accessions do not require a title, but browsing Collection Management records breaks if there is a Collection Management record that belongs to an Accession that has no title.

This is fixed by looking for display_string if there is no title.

There's also a selenium test to exercise the bug.