Fix message header

What does this MR do?

• moves avatar url calculation out of the API (see scripts.js::img)
  • fixes avatar display in message page chat header
  • (this behaviour was recently unified and broken by RestNormalization)
• changes a leftover reference to searchindex.json to use API (for autocompletion of members when creating new conversations)

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

Tested locally

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))