Resolve "Changed firm - Old firmname shows up in Messages"

Closes #294 (closed)

What does this MR do?

Everytime a store is updated, the depending conversation names are also updated.

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

Should not be broken when deployed; uses two more SQL-instructions.

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))