Render directly embedded image data represented as a string in Jupyter notebooks

Summary: Depends on D19259. Ref T13105. Some examples represent image data as ["da", "ta"] while others represent it as "data". Accept either.

Test Plan: Rendered example notebooks with both kinds of images.

Reviewers: mydeveloperday

Reviewed By: mydeveloperday

Maniphest Tasks: T13105

Differential Revision: https://secure.phabricator.com/D19260