Update tex to include load-mathjax rather than rely on global
Update tex to include load-mathjax rather than rely on global

Summary: Please note that the linter complained about the order of a few methods so I had to move them around.

Test Plan:
Check out some fixtures that use tex.jsx such as
`texified-code.jsx.fixture.js` and ensure everything renders properly.

Reviewers: #webpack, john

Reviewed By: #webpack, john

Subscribers: john, kevinb, davidflanagan

Differential Revision: https://phabricator.khanacademy.org/D49193

Khan/react-components,Khan/react-components,Khan/react-components