Mark threads as closed

Closes #724 (closed)

What does this MR do?

Adds the possibility of closing a thread, so nobody can answere anymore. Each thread has a status field, which for now can be open or closed, but this can be extended later.

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

Not very sure about the frontend

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 usermoderation1@example.com
3. Go to an existing forum thread or open a new one
4. Click the new button to close it
5. The answer box should disappear and the title should change
6. Click the button again to reopen the thread
7. The answer box should appear and the title should change again
8. Check that the button is not visible for other users (e.g. user2)

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

(A short text that will appear in the release notes and describes the change for non-technical people. Not always necessary, e.g. not for refactoring.)