Add button that marks emails as unread

What does this MR do?

Adds a button to the email frontend that marks emails as unread and a REST endpoint for that matter. Someone asked for this in a support email (https://foodsharing.freshdesk.com/a/tickets/538) and I think it is a useful feature that most email clients have, too. When clicking the button, the email is marked, the popup is closed, and the status in the mailbox list is changed.

The frontend part could also be done differently, if you have any ideas, but I think the whole email frontend needs to be replaced with something better anyway.

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

Very sure

How to test

Steps a reviewer can take to verify that this MR does what it says it does e.g.

1. Checkout branch locally
2. Login as someone with a mailbox (e.g. userbot or storemanager1)
3. Go to your mailboxes (http://localhost:18080/?page=mailbox)
4. Open an email that is marked as read
5. Click "Mark as unread" -> the email should be marked as unread in the list
6. Refresh the page -> the email should still be marked as unread

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
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • 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.

Release notes text

In der Anzeige von E-Mails gibt es einen neuen Knopf, mit dem die E-Mail wieder als ungelesen markiert werden kann.