Fix highlight.js

Summary:
I broke highlight.js when I upgraded to 8.0. This adds a smaller set of
languages and uses the default solarized_light theme, but adds a bunch
of other themes if we want to play with them.

Test Plan: Loaded the test post, saw that things were properly highlighted.

Reviewers: bryan.hughes, endenizen

Reviewed By: endenizen

CC: jose.blanco, amie.kweon

Differential Revision: https://phabricator.rdio.com/D11123