Store html version for fast preview

[aplavin]

When one opens a pluto notebook, he doesn't see any output until all packages are installed and notebook executed. This takes lots of time, especially when looking through many notebooks to find something.

Would be real nice if Pluto automatically stored an HTML export-like version of each notebook in some cache directory, and show all outputs immediately for notebooks that were already executed on the computer. Replacing those outputs with updated ones when they become available during execution.

[greimel]

I started to use static-preview for all my repositories. If I am looking for something, I browse the repo's gh-pages.

[aplavin]

For GH repos it's nice, I'm mostly concerned about local notebooks.

[fonsp]

The use case here is partly resolved by #2563 ! Though it might still be cool to see cached old results in Safe preview mode.