Allow toggling comment confidentiality

NOTE: The proposal/way forward for this still needs to be defined. When trying to deliver we hit a number of blockers- see MR !66918 (closed)

Now you can add a confidential comment, but cannot change the confidentiality of a comment after it has been added. Only the first comment in a thread should be "toggleable" and toggling it should update the confidentiality of all comments in the thread.

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