Migrate to prebuilt extension and upgrade to jlab

[telamonian]

The prebuilt-extension build in this PR is working, but there's a couple of things to still address:

• leaky css
  • the css of the underlying h5web package has a filter: invert() under a plain body selector. Not only breaks all jlab colors, but somehow also breaks the sizing/geometry of the jlab main area widget
  • lots of other css with overbroad selectors
• the h5 widget itself renders fine, but for some reason the actual histograms aren't rendering
  • It seems like instead of rendering it's getting stuck in some loop that burns 100% cpu and locks up the browser interface

There's also a couple of minor things, maybe out of scope for this PR:

• update binder build to reflect new extension build
• update the various READMEs with the new build

[telamonian]

The leaky css should probably be dealt with upstream in h5web. One easy way to do it would be to use .less instead of .css files. Valid css is valid less, but less also supports nested selectors. So you could easily wrap all of h5web's selectors with a leading .h5web-root or something:

.h5web-root {
  
  ...all of the existing h5web css...

}

[loichuder]

Thanks a lot for the PR and the leads for the fixes, Max !

Don't be surprised if I am slow to go through it: there are things that I need to process and I need to decide how we will proceed for the update to JLab3 as we are using JLab2 at ESRF

[loichuder]

Closing as it is now part of #50