[dagit] Include credentials on GraphQL playground
Summary: Include credentials on the GraphQL playground to ensure that cookie value is passed along in the /graphql request.
Test Plan: yarn build-for-python, run dagit. Load /graphql, verify that requests occur successfully, with cookie info included.
Reviewers: dgibson, bengotow
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D8950