Let users signup without leaving the page they are on

[teolemon]

I had to re-log into GitHub after a computer issue, and was presented with this modal when I mistakenly clicked "Signup".
Being able to:

• Signup without leaving the page
• Being redirected to the page you were on (or the page you were trying to reach, like edit mode) would be nice.

Part of

• 🐣 Road to contribution / First time experience #5708

[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