This removes the N+1 queries caused by
`Api::Admin::OrderSerialier#ready_to_capture` when used from
`Api::OrdersController#index`. While it's fine for the single-order
controller actions, it's not for this one that deals with a collection
of orders.
Fortunately, `SearchOrders` is used only in this controller action so we
can put the `includes` calls there, otherwise, we would need to refactor
it a bit to pass in a context-specific AR relation.
Prior to this change, there was no way of knowing how much was owed (or
in need of refunding) at a glance for each order on the orders index page,
this information was only available by clicking through to an order
payment page
This change adds the outstanding balance for each order
