Change the scriptstyle size from 2/3em to 0.7em

Summary:
I don't know why we originally used 2/3em for scriptstyle, but both TeX
and MathJax use 0.7em for the scriptstyle size.

Test Plan:
- Look at huxley tests, make sure everything that changed is due to the change
  in font size

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D12869