added missing fields in items
SpinnerView not being used
annotate, updated serialiser and added fields to client-side model
bugfix
this method implementation should be overridden by the classes that include it
added order_item relationship
this router should return a new instance of venta to work with
view should be (someway) like this
OrderItemFormView overrides `itemSelected`

`itemSelected` is handled on this view. It creates a new `orderItem`
record and assigns it to `parentView`’s model
VentasNewView doesn't include `ItemOnFormMixin` anymore. And needs a model binding.
OrderItemView handles the view of order_items in forms

template incomplete
this template needs to be written yet
added OrderItem and Compra