It's been awhile since we have supported IE11 so it should be safe to remove
IntersectionObserver now.
From a TODO task in this repo:
> drop when we eventually drop IE11
Announcement of when we removed IE11 support:
https://meta.discourse.org/t/137984/40?u=blake
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
