[html-formfu] Model-DBIC fix

[Carl Franks]

Previously, when creating a new relationship record, Model-DBIC was
using create_related(), then setting the column values, then updating
the record.
This caused a fatal error when the related table had any NOT NULL columns.

I've fixed this by instead using new_related().
I've also changed most of the table columns used by the test suite to
NOT NULL, to ensure this bug doesn't come back.

Carl