Improve performance in big notebooks

Cells that are not visible should not be rendered at all in the browser.

This way, we should be able to have a notebook with a billion cells (ignoring the issue of scrolling to a specific one...), because all that are not visible would be paged out to disk and represented by a single number describing their height (if that).

No files, tests, or entry points are named. Start by clarifying the notebook rendering and paging design described in the issue, including how off-screen cells are represented. Done means off-screen cells are not rendered while visible cells remain usable at very large cell counts.

Written by the indexing model from the issue text.