Fix flag modals to improve accessibility

[Stef-Rousset]

🎩 What? Why?

This PR updates flag modals to improve accessibility, by adding:

• a fieldset and legend on radio buttons
• a value to the for and id attributes on radio buttons
• a value to the for and id attributes of the textarea

This PR is issued from the audit of Angers city (page 92 and 93), and corresponds to criterias 1.3.1 and 3.3.2 from WCAG

📌 Related Issues

• Related to Github card https://github.com/orgs/OpenSourcePolitics/projects/26/views/1?pane=issue&itemId=112542787&issue=OpenSourcePolitics%7Cintern-tasks%7C40

Testing

1. As a logged in user, go to a process, and then to proposals
2. Find a proposal with comments and report one comment
3. Right Click + "Inspect", and see in the html that the changes above are effective.

📷 Screenshots (optional)

Radio buttons

Textarea

[github-actions]

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[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions. @carolromero & @xabier feel free to chime in.