Add a result field to tasks

[mdellweg]

The way we currently need to infer what a task that is basically a deferred api view actually did for us is weird.
We should use the serializer from the viewset that dispatched the task and serialize the result into the tasks "result" field.
This would make the "typical task" much more like a "deferred view". (scheduled tasks can still leave the result empty).

This relates to:
#2028
#3771

[quba42]

Does this mean users waiting on a task result will no longer need to parse the hrefs in created_resources to figure out what type of things were created?