Performance - delay creating codemirrors

[fonsp]

When you load a notebook, it creates a codemirror for every cell. We could optimise this by only creating codemirrors for cells that are not folded.

Cells that are initially folded should only create the codemirror when it is first unfolded.

Maybe we can even delete codemirrors when they are not visible? Instead of the current display: none

[novoselrok]

Additional enhancement would be to initialize code mirrors only after they come into the viewport (using IntersectionObserver API).

[fonsp]

Solved by #484.

Strangely, the performance benefit is not noticeable, so let's not do the viewport thingy