Use contenthash instead of chunkhash for better long-term caching (#6387)

Author: ianschmitz

packages/react-scripts/config/webpack.config.js

@@ -161,11 +161,11 @@ module.exports = function(webpackEnv) {
       // There will be one main bundle, and one file per asynchronous chunk.
       // In development, it does not produce real files.
       filename: isEnvProduction
-        ? 'static/js/[name].[chunkhash:8].js'
+        ? 'static/js/[name].[contenthash:8].js'
         : isEnvDevelopment && 'static/js/bundle.js',
       // There are also additional JS chunk files if you use code splitting.
       chunkFilename: isEnvProduction
-        ? 'static/js/[name].[chunkhash:8].chunk.js'
+        ? 'static/js/[name].[contenthash:8].chunk.js'
         : isEnvDevelopment && 'static/js/[name].chunk.js',
       // We inferred the "public path" (such as / or /my-project) from homepage.
       // We use "/" in development.