OrderAssignEvent doesn't have access to the original customer/email

    $order->setCustomer($account);
    $order->setEmail($account->getEmail());
    // Notify other modules.
    $event = new OrderAssignEvent($order, $account);
    $this->eventDispatcher->dispatch(OrderEvents::ORDER_ASSIGN, $event);

Our current flow is:
1) Modify order
2) Dispatch event
3) Save order

That means that event subscribers aren't able to determine the original customer/email. This is often needed, some actions are only performed when assigning an anonymous order (VS reassigning an authenticated one). This oversight is a result of OrderAssignment initially only being used for anonymous orders, but that's no longer true today (due to OrderReassignForm).

Plan

1. Clean up the service and event by replacing $account with $customer, deprecating the old event getter.
2. Fire the event before the order is modified.
3. Fix the event docs to mention in which cases the event is fired, and what the order contains