Avoid client side templating

[zcorpan]

I would really prefer that we write the content directly into index.html during merge-data instead of loading the JSON at runtime and client side templating (to avoid some of the workarounds for fragment functionality etc). That said, we can work through that in followups, and this is overall a big simplification for the frontend (including removing many dependencies)

Originally posted by @bgrins in #1063 (comment)

[bgrins]

And to be clear, I think we should also keep the json file around to make it easy to consume from other automated tooling. So we'd write JSON just like we do today, but update that script to also template out HTML to inject into index.html.