Moderation - failing test

See
https://github.com/mysociety/fixmystreet/blob/report-moderation-candidate/db/schema.sql
and
https://github.com/mysociety/fixmystreet/blob/report-moderation-candidate/perllib/FixMyStreet/DB/Result/Problem.pm
(in latter, 'cascade_delete => 0' is inferred by SL, but overridden
below)