message: allow read receipts without an existing member in MessagePanel
diff: @@ -560,9 +560,6 @@ module.exports = React.createClass({
  return; // ignore ignored users
  }
  const member = room.getMember(r.userId);
  - if (!member) {
  - return; // ignore unknown user IDs
  - }
  receipts.push({
  userId: r.userId,
  roomMember: member,
  