[dagit] Rename auth header (1/n)
Summary: The authorization header was causing conflicts so I am proposing a rename to Dagster-Custom-Auth-Token.
Test Plan: spun up dagit, confirmed that Dagster-Custom-Auth-Token header is sent if headerAuthToken prop is set
Reviewers: dish, dgibson, sashank, max
Reviewed By: dgibson
Differential Revision: https://dagster.phacility.com/D7869