IB TWS: add support for Advisor / Friends & Family Accounts

I have a Friends & Family account at IB because I manage multiple accounts.

I had no problems connecting to the IB TWS Gateway with the Zorro plugin, specifying a user’s account number in Accounts.csv while logged in as a Friends & Family advisor. The balance information appears correctly.

But there's one problem: If I place an order, I get this error:
!Order rejected - reason:Invalid or missing IBCustAcctNo AIV-STK--0.00--SMART/NYSE--USD

I assume that this field is normally left blank for single-user accounts. Apparently you need to be more specific than that if you manage multiple accounts from a single endpoint.

Oh, in fact I had the wrong account number in Accounts.csv, so it turns out orders are working now. Please disregard this post.