[#81628184] User is added as participant when commenting

When you add a comment, if you are not a participant you are added as a participant (assuming you have the permission to comment). Also, your avatar will appear in the list of participants after commenting (thanks event stream!)
I would leave `"creates a new comment"` and `"returns the new comment as json"` outside of the `"the user is a participant on the card"` context. Those two things happen in both contexts.