Fix meeting registrations questionnaire free text choice answers export

[ahukkanen]

🎩 What? Why?

When the meeting registrations form has a question with single or multiple choices and they have the free_text option defined for them, those free text answers won't be exported.

This fixes the issue by utilizing the Decidim::Forms::UserAnswersSerializer serializer to export the registration form answers.

Testing

• Create a meeting with a registration form
• Add a question to the form with the single choice or multiple choice types
• For one of the answer options, enable the "free text" option
• Submit an answer to the form with a free text choice answer
• Export the meeting registrations from the admin panel
• See that the free text answers are not exported correctly

📋 Checklist

• ❓ CONSIDER adding a unit test if your PR resolves an issue.
• ✔️ DO check open PR's to avoid duplicates.
• ✔️ DO keep pull requests small so they can be easily reviewed.
• ✔️ DO build locally before pushing.
• ✔️ DO make sure tests pass.
• ✔️ DO make sure any new changes are documented in docs/.
• ✔️ DO add and modify seeds if necessary.
• ✔️ DO add CHANGELOG upgrade notes if required.
• ✔️ DO add to GraphQL API if there are new public fields.
• ✔️ DO add link to MetaDecidim if it's a new feature.
• ❌AVOID breaking the continuous integration build.
• ❌AVOID making significant changes to the overall architecture.