Multi-PUT, order of events is uncontrollable and can mess with model validation

[stam]

In TMS we have an Order model which has multiple Activities.
In the Order model, we define some constraints about the Activities.

When we do a MultiPUT on /api/order, we cannot control the order in which the models are saved.
It can occur that the Order model is saved before the Activity models, this causes Order.clean to be fired prematurely: the order.activities will contain the old version of the activities.

When the activities themselves are saved, the Order.clean isn't called afterwards and we are left with data which is invalid by our constraints.

A failing testcase of this behavior can be found here:
https://bitbucket.org/codeyellow/tms/src/ed1f0b7c4ef3071911b856a9ea47abf9d8818385/backend/commerce/tests/test_order.py?fileviewer=file-view-default#test_order.py-177