[dagit] React.lazy pipeline playground
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