[Feature Request] Add ability to download and host widgets locally in a deployed grist container

[josh-hemphill]

Trying to reduce the amount of external urls and when trying to get it to work for myself, it's a little bit of a pain to do at the moment, so I thought I'd ask about having something added to make it easier. I was thinking it might be possible to include zips in the releases of all the publicly served bits with relative URLs.

With something like that, eventually grist-core could have something to download it to the static folder, it would require another volume to be attached to the container, but it seems the most straight forward solution.