Refactor render-asset-snippets.js

[davidwarrington]

render-asset-snippets.js was written as a JS file because I thought it would be really simple, but it's turned out to be more complex than I'd hoped, and less readable as a result. It's also stuck between trying to remain independent of the layout/template differences (for example using {% if layout == 'theme' %} vs {% if template.name == 'product' %}) and just addressing them directly.

I'm expecting Sections Everywhere to either make the entire entrypoints strategy need a complete rethink, or make this module require a complete rewrite anyway. So this isn't a major priority right now.