Add a plain text alternative to HTML emails such as the welcome email for new users

[stephanegigandet]

What

• It used to be a good thing (for accessibility, and possibly deliverability) to also send a plain text alternative to HTML emails using MIME multipart/alternative. The new welcome email is only in HTML (fix: Update welcome email #8703)
• For what it's worth, the Brevo newsletter we send always include an automatically generated plain text alternative.
• Maybe we could just strip HTML tags and inline links URLs to generate an alternative email

Part of

• 📨 Email engagement (tracker) #5730

[github-actions]

This issue has been open 90 days with no activity. Can you give it a little love by linking it to a parent issue, adding relevant labels and projets, creating a mockup if applicable, adding code pointers from https://github.com/openfoodfacts/openfoodfacts-server/blob/main/.github/labeler.yml, giving it a priority, editing the original issue to have a more comprehensive description… Thank you very much for your contribution to 🍊 Open Food Facts

[alexgarel]

https://metacpan.org/pod/HTML::WikiConverter::Markdown could be a good tool.