added noopener rel to external links on main page.
What does this MR do?
How confident are you it won't break things if deployed?
no danger from this
- 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))