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Handle m2m prefetching in polymorphic (#3035)

To properly handle m2m prefetching of polymorphic objects (ex. `Operation.objects.prefetch_related(Prefetch(BaseObject.polymorphic_objects.all()))`), the following steps had to be added to polymorphic iterator:
* copying extra fields from base queryset
* copying filters from base queryset, to properly handle joins - this could be optimized in the future to not add unnecessary `WHERE` statements to SQL query.

* Fix `polymorphic_objects` in prefetch related, when base model is filtered

Based on example with Operation and BaseObject: it happened,
that when Operation objects were filtered, `polymorphic_filter`
still contains `Operation.objects.all()`, so all BaseObjects with
particular id(s) which have corresponding Operation (any! not only
the ones filtered in base query) were fetched from DB - but (here
comes the magic!) we were iteration through PKs of objects (BaseObject)
from original query and yield them in the same amount as they occur in
original query (so if there were 3 BaseObjects with PK 1234 in
original query, a lot more objects could be fetched, but only 3
of them were returned, and they might not correspond to current query
of base model (Operation)).

Right now, every fetched object is returned.

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