Add default value for 'expired' field at the database level

0004_fields_for_scheduled_publishing created a new BooleanField called 'expired' but didn't
set the default value at the database level.

This caused issues when datamigrations written for older versions of Wagtail created pages as
they are not aware that this new field needs a value to be set.

This migration sets the default value into the database so it gets set automatically.