[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