Delete user rest

Touches #472 (closed)

What does this MR do?

• Introduce REST endpoint /api/user/{id} for DELETE to delete a user
• Use that endpoint for user deletion

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))