Allow pre-fetching of css files?

[AlexanderRichert-NOAA]

As far as I can tell, nbconvert cannot be built on non-internet connected systems, such as in an automated build system on an airgapped node. In hatch_build.py, it tries to download each css file (on at least one system I'm working on, the urlopen(url).read() function hangs indefinitely) before checking for an existing copy. Would it be possible to check for the destination file before trying to download it? In my use case (installing through Spack), it's fine to stage the files in advance so they're available at install time.

[blink1073]

Hi @AlexanderRichert-NOAA, that sounds reasonable, would you be able to submit a PR?

[AlexanderRichert-NOAA]

@blink1073 yes, please see #2095