Extension upgrades

[nicolaskruchten]

Here's a grab-bag of /packages/javascript stuff that would be good to do all at once:

• merge Added lazy loading to jupyterlab-plotly #2260
• apply the Added lazy loading to jupyterlab-plotly #2260 changes to https://github.com/plotly/jupyterlab-chart-editor
• move https://github.com/plotly/jupyterlab-chart-editor into this repo
• convert all bundles away from using the .min bundle, for faster build times
• move the process to using Node 12 instead of Node 8 (change of format to package-lock.json)
• wipe away package-lock.json and regenerate it with all-latest versions
• change readme to de-emphasize the widget-related installation steps and the memory-related installation mitigations in the "main line" installation instructions