R1:df139f044b9b

Render proper "Show Context" links in DocumentEngine diffs, not just bullets

Summary:
Ref T13513. Currently, viewing a Jupyter document, hidden context just gets a plain "* * *" facade with no way to expand it.

Support click-to-expand, like source changes.

Test Plan:
- Clicked to expand various Jupyter diffs.
- Clicked to expand normal source changes.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21243