JavaScript cleanup

[duckinator]

I noticed there's a lot of room for improvement regarding our JavaScript usage.

Drop IE8 support

IE 10 support ended in January 2025 — half a decade ago — so I don't think we should bother supporting things that are even older than that.

Dropping IE8 support means we can remove the following:

• modernizr
• selectivizr
• html5shiv
• ie8.css

Avoid using JS for navigation

Our navigation on mobile uses JavaScript. We should migrate to a CSS-only implementation — it can be done more simply, and without JavaScript.

Split out to #214.

Avoid loading things twice

For some reason, application.js is loaded twice. We should stop doing that.