Update step indicator total pages when a user forks

[JoeChapman]

When a user forks it can change the number of remaining pages in their journey. I would like to be able to reflect this in a step indicator view parameter

[JoeChapman]

@UKHomeOffice/hof after some chat and https://www.gov.uk/service-manual/user-centred-design/resources/patterns/progress-indicators.html#section-3 I think we should conclude that this is not a thing we should be concerned with. Instead where journeys lengths may be dynamic a different solution for indicating the users' whereabouts should be designed

[joefitter]

After further discussion the initial solution will be to display the index of the current step, but not show how many steps there are in total (as this may change). This should take into account re-visiting a step, and when steps are invalidated (it shouldn't skip)