Revert "Revert "Metacognitive experiment.""

This reverts commit 0ebd9e55b658a84f30d8f2c596dd216ca128c8b2.
changes in response to i18n/lint errors.  (also see https://sites.google.com/a/khanacademy.org/forge/for-developers/i18n#TOC-Marking-up-Natural-Language-in-khan-exercises-Files )

Summary:

Test Plan:

Reviewers:

CC:
Merge branch 'master' of https://github.com/Khan/khan-exercises