[MM-58726] Add a loading screen to initial load of the web app (#27740)

webapp/channels/webpack.config.js

@@ -133,7 +133,7 @@ var config = {
             'mattermost-redux': 'packages/mattermost-redux/src',
             '@mui/styled-engine': '@mui/styled-engine-sc',
 
-            // This alias restricts single version of styled components acros all packages
+            // This alias restricts single version of styled components across all packages
             'styled-components': path.resolve(__dirname, '..', 'node_modules', 'styled-components'),
         },
         extensions: ['.ts', '.tsx', '.js', '.jsx'],
@@ -195,6 +195,7 @@ var config = {
                 {from: 'src/images/payment_processing.png', to: 'images'},
                 {from: 'src/images/purchase_alert.png', to: 'images'},
                 {from: '../node_modules/pdfjs-dist/cmaps', to: 'cmaps'},
+                {from: 'src/components/initial_loading_screen/initial_loading_screen.css', to: 'css'},
             ],
         }),