Use a list input on page 21, question 1

Previously this question had a one-line textbox. It should be a numbered list
input instead, to match what's in the content. I think this page got missed
because it's only in the SaaS flow, not the IaaS flow.
Merge pull request #55 from alphagov/list-input-p21

Use a list input on page 21, question 1