Update Welsh CSV service for multiple exit pages

[lfdebrux]

What problem does this pull request solve?

Trello card: https://trello.com/c/Qe1U2bBO/3181-change-welsh-translation-feature-to-work-with-multiple-exit-pages

When the multiple branches feature is released, the Welsh translation CSV should include the number for each exit page in the row heading, so that exit pages can be uniquely identified for a form when there's a page with more than one exit page.

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[thomasiles]

nice 🏅