ActivityPub: Change addressing of Undo.

Undos used to address the followers and the author of the undone message, which doesn't make much sense because the author who undoes is always the author of the original activity. It should instead address the author of the activity that the original reaction reacted on (that is, the author of the liked / announced post).

This MR does just that.

In general, I'll try to unify undo once the emojireaction MR is merged.