Add a requirejs shim for highlight.js

Before this, highlighting code in markdown cells was broken since hljs wasn't available.

A better solution might be to build the highlight.js amd module (i.e., build the highlight.js module with `--target=amd`) so that we don't need the shim.
Move shim to the standard location
Merge pull request #6188 from jasongrout/highlightjs-require-shim

Add a requirejs shim for highlight.js