Fix accessibility in footer

[Stef-Rousset]

🎩 What? Why?

This PR improves accessibility in footer, by:

• replacing a meaningless div by a p
• adding new translations for Social media and Help
• adding more explicit text in alt attribute on logo

This PR comes from the audit of Angers city (pages 32, 33 and 34), and refers to criterias 1.1.1, 1.3.1, 2.4.4, 2.5.3 and 3.1.2 from WCAG.

📌 Related Issues

• Related to https://github.com/OpenSourcePolitics/intern-tasks/issues/80 https://github.com/OpenSourcePolitics/intern-tasks/issues/79 https://github.com/OpenSourcePolitics/intern-tasks/issues/77

Testing

1. As a user, go to the footer of the page and open dev tools
2. Check that the alt attribute of the organization's logo img has a "Back home" displayed (screenshot one)
3. Check that the Help title and that the aria-label="Social media" on nav links are well displayed (screenshot two)
4. Check that the div displaying the text "Website made with..." has been replaced by a p

📷 Screenshots (optional)

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