Summary: Another nice little bundle splitting win, one that I had tried to do before: load the pipeline playground lazily. This splits CodeMirror/ConfigEditor out.

Test Plan: Run dagit locally, verify that pipeline playground renders. Repeat with path-prefix after `yarn build-for-python`, verify same.

Reviewers: bengotow, prha

Reviewed By: prha

Differential Revision: https://dagster.phacility.com/D7510