Handle paginated responses better

[kdmccormick]

Sometimes, drf-yasg can't figure out the appropriate serializer for a class. That's not a problem if we have a serializer on hand to pass to schema:

@schema(responses={200: MySerializer})
def get(...):
    """ My view """

For paginated views, this becomes a problem, because we don't have a paginated serializer to give schema.

I see two solutions to this:

1. Help drf-yasg figure out paginated serializers better

Why can't drf-yasg generate a correct paginated response schema? Are we missing a class variable on our views? Are we using DRF wrong?

2. Provide a manual paginate_serializer function as a fall-back

In the case where drf-yasg just can't figure out the response schema, it'd be great to be able to do something like:

@schema(responses={200: paginate_serializer(MySerializer, MyPaginationClass)})
def get(...):
    """ My paginated view """

[kdmccormick]

This PR would provide paginate_serializer for edx-platform, but it'd be better to add this at the api-doc-tools level.