mailbox follow up

Part of #1668

What does this MR do?

Fixes:

• If you answer and email and type in a new recipient, the new recipient is being ignored.
• TypeError in selectedMailbox (src/components/Mailbox/MailboxView): Cannot set properties of undefined (setting 'isBusy')
• If an incoming email was sent to multiple recipients, they are not shown in the frontend. You can only see yourself as a recipient.
• Wrong German translation in mailbox->mailsend_unsuccess: E-Mail konnten -> E-Mail konnte
• within a mail, all recipients are now displayed in the to field. if there are too many, they can be expanded using the more button.

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

Links to related issues

How to test

Screenshots (if applicable)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• added to the next milestone (see https://gitlab.com/foodsharing-dev/foodsharing/-/milestones, unless it has a "for:Dev" label)
• added an entry to CHANGELOG.md
• added a short text in the release notes to /release-notes/YYYY-MM.md
• Once your MR has been merged, you are responsible to create a testing issue in the Beta Testing forum: https://foodsharing.de/region?bid=734&sub=forum. Please change the MRs label to "state:Beta testing".
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...) how this change can be tested.
  • Be aware, that also non technical people should understand.