Request birthdate and age 18+ on registration

Closes #196 (closed)

What does this MR do?

The MR

• Asks the user for a date on registration
• Verifies the age to be 18+

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

Tests work...

Links to related issues

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))