Integrity Checker should be updated to check for deleted custom fields in FieldConfiguration

There is a bug which causes the database to be in an inconsistent state after deleting custom fields. There should be additional checks in the Integrity Checker to identify these particular inconsistencies. Specifically, when deleting a Custom Field, any associated FieldConfiguration rows which contained that Custom Field in the FieldId column are not deleted. This causes problems when trying to retrieve those FieldConfigurations later, as the Field they refer to no longer exists.