Allow to mark conversation as unread

[nickvergessen]

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Steps to reproduce

1. Have a chat with messages
2. Open it and scroll to the end

Expected behaviour

• Show an option Mark as unread
• Calls the new API: Allow to mark a conversation as unread #6807
• Switch away from the conversation if open, to not mark it as read again with the next message polling

Actual behaviour

Still see "Mark as read" option in the left sidebar which is not functional

Talk app

Talk app version: master