Convert another batch of smart answers to use new next_node syntax

Converts another batch of smart answers to the new syntax using `next_node_if` to define next nodes.

* overseas-passports
* maternity-paternity-calculator
* register-a-birth
* register-a-death

This will allow helpful automatic visualisations of all of these flows.

This has been rebased against master and I've avoided modifying any flows which have been modified on master since the `next_node_if` code was merged, so it should merge cleanly.