Only specify devServer options for first webpack config

This avoids `[webpack-dev-middleware] ConcurrentCompilationError`
issues when running `webpack serve`. However, it prevents running the
dev server for just the `wall-display` config (as it no longer has the
`/calendar` proxy route defined.

webpack.config.js

@@ -2,14 +2,6 @@ const path = require('path');
 
 const common = {
   mode: 'development',
-  devServer: {
-    proxy: {
-      '/calendar': {
-        target: 'https://calendar.google.com',
-        changeOrigin: true,
-      },
-    },
-  },
   module: {
     rules: [
       {
@@ -38,6 +30,15 @@ module.exports = [
     ...common,
     name: 'default',
     entry: './src/index.ts',
+    devServer: {
+      allowedHosts: 'all',
+      proxy: {
+        '/calendar': {
+          target: 'https://calendar.google.com',
+          changeOrigin: true,
+        },
+      },
+    },
     module: {
       rules: [
         {