#386 Post-Python 3: Unmerge submissions can contain deleted pubs

[Ahasuerus]

Unmerge submissions can contain deleted publication records. It can only happen if publication records are deleted between the time the "Title Unmerge Request" page is loaded and the time "Submit Unmerge" is clicked. It's not a significant issue because the submission review page catches this problem and forces a Hard Reject error, but it would be nice to have it addressed.