Apply requested ordering to only to new cursors

[annehaley]

This issue in Sentry has been occurring since the introduction of the PinnedFirstPagination class. The error ValueError("Cursor position does not match the current ordering.") is raised when an existing cursor is later passed pin_sort=True. This PR adjusts the logic in paginate_queryset such that we only adjust the ordering if we are creating a new cursor (if pagination.cursor.position is None). If the cursor already has a position, continue with the same ordering regardless of whether pin_sort=True is present in the current request.

This PR also adds a test to confirm that the error is no longer raised if we try to change the ordering on an existing cursor.