Moves Highlight.js files to vendor/assets/javascripts
Adds Highlight.js in yarn package management
Removes old rake task and reliance on NPM to build Highlight.js
Highlight.js is now integrated in the "javascript:update" rake task
This feature is used for defer loading of images and in future for post cloaking
This gives us a polyfill so we can safely use the feature in problem browsers
The polyfill supports "polling" but it does not appear we need it yet.
If we discover anything odd here, consider setting poll interval per:
https://github.com/w3c/IntersectionObserver/tree/master/polyfill
```
var io = new IntersectionObserver(callback);
io.POLL_INTERVAL = 100; // Time in milliseconds.
```
Keeping the mutation observer cause we often mutate the DOM
