Option to disable writing to file

[fonsp]

In a couple of situations, it makes sense to not save the notebook file after opening it. For example:

• on https://github.com/JuliaPluto/PlutoSliderServer.jl we don't want to modify the notebook files during the export process. For example, Pluto changes the second line to the current Pluto version number, which causes changes to lots of files.
• on binder, disabling file writing would be an optimization, since users always use the "Export to Julia file" option to download the file, or Built-in browser local file saving in Chrome #833

I never tested this code, anyone should feel free to:

• try it out
• fix any bugs
• merge main with this branch
• add a basic test