Explicit sequence name fix proposal

In relation to an issue discovered with the form app incorrectly redirecting when using PostgreSQL, this proposed fix adds a catch to the `DB::last_id` setup in the model class for postgres that explicitly sends a sequence name that follows the default format of the `serial` datatype: `tablename_fieldname_seq`.

After reinstalling the form app using it's proposed SQL setup, the redirect worked correctly with this fix.