Use contenthash instead of chunkhash for better long-term caching

Created by: ianschmitz

This changes the .js webpack file name values to use contenthash instead of chunkhash as recommended by the webpack team:

https://github.com/webpack/webpack.js.org/issues/2096 https://webpack.js.org/guides/caching/#output-filenames

This should improve long-term caching for our users by relying only on the contents of the .js file itself instead of the chunk as a whole (.js, .css, etc) to generate the filename.