Yet another mailbox follow-up

Part of #1668

What does this MR do?

• When you open the mailbox page from the menu, the first mailbox is selected by default
• Emails are marked as read by the backend. This makes the "mark as unread" button in the frontend work correctly.
• Use the eye and eye-slash icons for that button.
• Emails from the trash folder are not shown on the dashboard anymore.

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
• set the "API change" label if changes in the API are not backward compatible
• 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.

Closes #1668