[dagit] Enable limited frame-src
As reported on Slack.
Re-enable frame-src CSP directive to allow showing ipynb iframes. Allowed on localhost for dev, otherwise 'self'. This has been broken since the CSP was released.
Run dagit with dagstermill repo:
$ dagit -p 3333 -m dagstermill.examples.repository
Use "View notebook" in job overviews to view the notebook dialog. Verify that there are no CSP issues, and that the notebook renders as expected, with appropriate styling.
Repeat with a prod build.
Reviewers: bengotow, max, yuhan, prha
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D9210