Use jekyll-assets for managing all assets

[tbatchelli]

https://github.com/ixti/jekyll-assets

We could use the bootstrap pipeline, minification and compression.

There are open questions, mostly they have to do with mini-sites and how they will have access to the new assets: currently assets live in their own CDN, and are manually versioned. So using this plugin it's not clear that
1- we can push them to the CDN -- we can probably do so if we don't use the liquid tags
2- other project can reliably reference these generated assets.

If we generate assets without cache busting and we don't use the jekyll-cache tags, then we should be able to continue pushing the assets are we're currently doing...