#389 Post-Python 3: 'Add Variant Title' loophole for REVIEWs/INTERVIEWs

[Ahasuerus]

The navigation bar on the left doesn't include the menu option to 'Add Variant Title' when the displayed title is a REVIEW or an INTERVIEW. However, you can get around this limitation by pulling up the "Add Variant Title" page (https://www.isfdb.org/cgi-bin/edit/addvariant.cgi?N) for another title record, then changing the numeric value in the URL to that of a REVIEW or an INTERVIEW title. We need to add a check preventing edit/submitvariant.py from processing 'Add Variant Title' submissions for REVIEW/INTERVIEW titles.