Hide questions that the user hasn't seen from the check your answers page

### What feature do you want to see added?

Hide questions that the user hasn't seen from the check your answers page. This page should only show questions that the user visited.

### Why do you want this feature? Does it solve a problem?

Currently the check your answers page shows all questions, even if the user didn't navigate to them due to branching. This could be confusing as the user might think they've missed a question, when they actually weren't supposed to answer it.

### How should the feature be implemented?

Can we calculate the path that the user took through the form, and use that to determine which questions to show?

Note that optional questions that the user didn't answer should still be included and shown on the check your answers page.

This feature will need to be added to the ZIP download too.

Consider if the logic used for dynamic back links (added in #112) can be reused for this.

### Additional context (if applicable)

This follows on from #41.