Preload assets

[frnsys]

Preload assets at a loading screen so that they aren't fetched on-demand (which causes them to flash in).

Things to look at:

• https://developer.mozilla.org/en-US/docs/Web/HTML/Link_types/preload
• https://github.com/vuejs/preload-webpack-plugin

The easiest way might be to specify what we want to preload in an array and then, using Vue, dynamically generate:

<link :href="path" rel=preload as=image>