Add instant.page for easy UI speed improvement

[david-a-wheeler]

The instant.page JavaScript is a tiny piece of JavaScript that causes preloading of a page just before the user selects it. The instant.page website explains it this way:

Before a user clicks on a link, they hover their mouse over that link. When a user has hovered for 65 ms there is one chance out of two that they will click on that link, so instant.page starts preloading at this moment, leaving on average over 300 ms for the page to preload.
On mobile, a user starts touching their display before releasing it, leaving on average 90 ms to preload the page.

The JavaScript is MIT-licensed. It doesn't work in some situations, but that just means it falls back to "normal operation".

This looks like something that would speed up interaction from a human point of view, even though our performance measures won't reveal any change. But it looks like a good idea for the humans :-).