Remove YUI Compressor and use plain JavaScript

[argilo]

YUI Compressor was introduced in 0fabf4d and b7105ca as a means of reducing the number of requests required to render the page. But it has some downsides:

• YUI Compressor was abandoned about a decade ago, and does not support modern JavaScript syntax.
• It does not generate source maps, which makes debugging difficult.
• It requires developers to have Java installed.

Since HTTP/2 is now widely supported and allows separate files to be multiplexed over a single TCP connection, we should be able to remove YUI Compressor and switch back to plain JavaScript without increasing load time.

To avoid the need to insert version information into JavaScript files, I moved the version information into data attributes in index.html's body tag.

Previously, build.sh updated index.html in place, but this made it difficult to work on that file. I renamed index.html to index.template.html, from which index.html is now generated.

The list of cached CSS & JavaScript files in service-worker.js had fallen out of date, so I updated that as well.