Make base/js/dialog conditional in webpack #298

Right now the nbextension and the labextension for qgrid are different.

The nbextension (installed via pip) contains (in local file nbextensions/qgrid/index.js) define(["@jupyter-widgets/base","base/js/dialog"].... Voila, when configured to server local nbextensions, will try to serve that local file, but since base/js/dialog is provided by the classic notebook and isn't there in Voila, you get an error. If you're offline, game over, qgrid won't work. If you're online, Voila will fallback to unpkg.com, in which case it's actually getting the labextension code instead, which then works.

This is coming in via https://github.com/quantopian/qgrid/blob/master/js/webpack.config.js#L63 which was added b3f8379#diff-69efad4a1399d8cd8a26a3771ae156df.

Is there a way for webpack to conditionally link to externals? We do a conditional requires at https://github.com/quantopian/qgrid/blob/master/js/src/qgrid.widget.js#L11. What would we check to make sure we can load base/js/dialog?

Longer discussion thread at: #291