[dagit] Fix manifest
Summary: The manifest currently fails to load in cases where auth is required on the backend, as credentials are not included in the request. Fix this, as indicated by https://developer.mozilla.org/en-US/docs/Web/Manifest#deploying_a_manifest_with_the_link_tag.
Test Plan: Load Dagit in prod build and dev build. Verify that the manifest request succeeds with no errors.
Reviewed By: bengotow
Differential Revision: https://dagster.phacility.com/D9030