upgrade to new version of ipywidgets

[williamstein]

See

jupyter-widgets/ipywidgets#3405

What we currently use:

~/cocalc/src/packages/frontend$ grep widget package.json 
    "@jupyter-widgets/base": "^1.2.5",
    "@jupyter-widgets/controls": "^1.5.3",
    "@jupyter-widgets/output": "^1.1.5",
    "@phosphor/widgets": "^1.9.3",

These above versions are pretty old, e.g., @jupyter-widgets/base is at version 4.0.0 now. However, I'm told that this doesn't actually break backward compat as much as the version suggests, since the version is bumped due to some compat issue with jupyterlab.

• Critical to be aware of backend version; make sure it is at least version 7.

[williamstein]

The frontend now uses the latest jupyter widgets via this package

https://www.npmjs.com/package/@cocalc/widgets

which is based on a heavily modified and updated version of https://github.com/googlecolab/colab-cdn-widget-manager

Also, cocalc no longer ships its own fork of k3d's frontend client. All custom widgets are pulled from the standard ipywidgets cdn (jsdelivr) with version requested by the backend, as is done by colab.