Add \enspace (0.5em space)

Summary:
TeX spacing is complicated - there's \enskip and \enspace and the two are
subtly different in some weird way. But \enspace seems to be the preferred
half em space in LaTeX, and the only one which MathJax supports.

Test Plan: Parsed and rendered correctly.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D6437