Make cards in app/views/discussions/_discussion.html.haml Pajamas-compliant

One or more cards in app/views/discussions/_discussion.html.haml are not Pajamas-compliant. See the parent epic for guidance.

Note: Threads/discussions don't seem to be using the correct .card structure. One fix for this issue would be to just remove the .card class altogether. Or we could fix the structure. Since commit threads use haml and MR threads use vue there's a bit of a disconnect here too, and the card noted in this issue has some UI bugs:

MR thread	Commit thread