Newsletter permission fix/finetuning

Bugfix/extension for: !1235 (merged)

What does this MR do?

Extend this permission to the xhr calls which are used in this class. I will add this merge request number to the existing changelog line

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

Very

Links to related issues

• !1235 (merged)

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Set mayAdministrateNewsletterEmail() in code to return true;
3. Login as ambassador
4. Open the newsletter page: http://localhost:18080/?page=email
5. Try if testmail is working
6. Try if newsletter to a group of people is working

Screenshots (if applicable)

Any relevant screenshots if this is a design / frontend change

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))
• Once your MR has been merged, you are responsible to update the #foodsharing-beta Slack channel about what has been changed here. They will test your work in different browsers, roles or other settings