Altered <pre> style tags so they have the class "code" Enabled horizontal rules Altered images to mimic how images are currently stored in html.

Test Plan:
Open editor and test buttons
See if https://www.khanacademy.org/science/projects/robots/Spider_bot/a/14-teaching-spider-to-dance https://www.khanacademy.org/humanities/art-history/art-history-400-c-e--ancient-cultures-1/buddhist-art/a/introduction-to-buddhism and https://www.khanacademy.org/coach-res/case-studies/oakland-unity/a/about-oakland-unity render properly when opened in ckeditor. Nonfatal error if it occurs.

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Reviewers: tom

Differential Revision: http://phabricator.khanacademy.org/D2726