Show progress bar earlier during load

[fonsp]

On very slow connections, you need to wait for the full editor JS to be loaded until you see the fetch progress bar.

This PR does some "server side rendering" to show an indeterminate progress bar immediately when the html and CSS loads, which will be "hydrated" when the editor JS is loaded.

Here is a "Fast 3G" throttled load of a featured notebook. Notice first the (greyed out) progress bar, replaced by the real progress bar, and once the statefile is loaded, the notebook. Before this PR, you would see a blank screen.

Schermopname.2024-04-09.om.12.57.32.mov

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="earlier-progress-bar-during-load")
julia> using Pluto