fix(threads): Notify clients inside a thread when the history was purged

[nickvergessen]

🛠️ API Checklist

• 1: Open a conversation with a thread
• 1: Open the thread
• 2: As a moderator clear the history of that conversation in another window
• 1: You should notice that something happens, in best case the client changes back to the normal chat where you can read that the history was cleared

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not possible
• 📘 API documentation in docs/ has been updated or is not required
• 🔖 Capability is added or not needed

[nickvergessen]

/backport! to stable33

[nickvergessen]

/backport! to stable33