Perform basic block interdiffs when diffing abstract blocks, and interdiff markdown in Jupyter notebooks

Summary:
Depends on D20844. Ref T13425. When we line up two blocks and they can be interdiffed (generally: they both have the same type of content), let the Engine interdiff them.

Then, make the Jupyter engine interdiff markdown.

Test Plan:

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20845