WIP: Only allow basket comments for basket providers

Closes #534 (closed)

What does this MR do?

Only allow basket comments for basket providers since it's falsely used to request baskets instead of via chat. Comments don't trigger a notification. The basket provider isn't aware of someone requesting it there.

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

Should work.

Links to related issues

• Slack discussion: https://yunity.slack.com/archives/C1T1ZM9A4/p1558634467010700

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))