Offline export should still try to download typefaces from the internet

[pankgeorg]

After https://github.com/fonsp/Pluto.jl/pull/2802 the HTML export will bundle pretty much all JS, but not the fonts (which are about 15MB). It will not try to fetch the fonts from the internet currently (as the links will be "stripped")

The best option for users is just to install the fonts on their system. But we should try to get the fonts from the internet even if the user doesn't have it locally, in a best-effort manner.