(Temporary) caching cdn handler until we sort out bundling

[maartenbreddels]

Issue to solve

For loading js modules, we have to choose a system, either the notebook extension method or the jupyter lab bundling. This new loader (frontend) will be a temporary solution that will 'just work' until we find the proper way to do this. It will try to fetch from <server>/voila/require/module@semver, which at the server will

• try to find the cached version and return that
• if not found will fetch it from a CDN (default is unpkg) and write it to cache

But only for modules in a whitelist (security concern)

Issues:

It will not respect semver, so will never fetch a patched version once it cached a version.

Where cache?

It first tries {prefix}/share/jupyter/voila_cache/{module}/{semver}, and on failure will use the user directory.

Note

This is based on #2, so that first needs to be merged, and this rebased.

[SylvainCorlay]

Could you rebase this PR on master to isolate the only change.