One of the pain points in developing Superset frontend code is the lack
of testing data. Local installation often do not have enough examples
setup to test all edge cases.
This change allows `webpack-dev-server` to proxy to any remote Superset
service, but the same time replaces frontend asset references in HTML
with links to local development version. This allows developers to test
with production data locally, tackling edge cases all while maintaining
the productivity of editing the code locally.
