Add hash-based cache busting for client side bundles

[Thomas-Geraghty]

• Added esbuild-plugin-manifest module for generating a manifest file that maps original asset name to its generated output, structured like below

{
  "/assets/js/app.js": "/assets/js/app.UG7VY7MS.js",
  "/assets/css/app.css": "/assets/css/app.47MYFHW7.css"
}

• Added logic to read manifest file and use mappings through a nunjucks filter called assetMap to swap asset strings for the build output
• Removed old cache busting code

[psoleckimoj]

LGTM!

[andrewrlee]

LGTM