Closes#5237
Generics/ModelViewset performs filtering on: list, retrieve, put, patch and destroy (plus method equivalents).
i.e. on list plus anything that calls `get_object`.
This PR makes schema generation follow that.
It adds `AutoSchema._allows_filters()` which can be overridden in subclasses.
I’ve made this initially “private” so we can make quick changes if needs be in a 3.7.1 etc.