Make basic Juypter notebook rendering improvements and roughly support folding unchanged context

Summary:
Depends on D20843. Ref T13425. Add very basic support for "Show Hidden Context", in the form of folding it behind an unclickable shield. This isn't ideal, but should be better than nothing.

Prepare for "intraline" diffs on content blocks.

Fix newline handling in Markdown sections in Jupyter notebooks.

Remove the word "visibile" from the codebase.

Test Plan:

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20844