added noopener rel to external links on main page.

What does this MR do?

https://developers.google.com/web/tools/lighthouse/audits/noopener

How confident are you it won't break things if deployed?

no danger from this

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))