Replace tex.jsx with react-components/js/tex.jsx

Summary:
The first changeset in phabricator is the unification
of these two files. The second is the deletion of
src/tex.jsx and replacement with react-components'
version

Test Plan:
Load perseus locally, bring up a transformer, expression, and interaction
(which use <TeX> for transformation outputs, buttons, and graph-settings canvas scale output)

Reviewers: emily, joel

Reviewed By: joel

Subscribers: joel

Differential Revision: http://phabricator.khanacademy.org/D13382