Fix become bezirk allows joining any region

Closes #48

What does this MR do?

• Move xhr_becomeBezirk into REST API controller
• Checks permissions when joining a region so people can only join regions that are meant to be joined
• returns no unused personal data in that response
• use fetch api in client when joining a region
• fixes unit tests for dev environment (backup_globals set to false for PHPUnit)

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

Did not test joining a region yet, that's why WIP

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))