chore(layout): optimize external script loading

layout/common/head.jsx

@@ -150,7 +150,7 @@ module.exports = class extends Component {
             <Plugins site={site} config={config} helper={helper} page={page} head={true} />
 
             {adsenseClientId ? <script data-ad-client={adsenseClientId}
-                src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js" async={true}></script> : null}
+                src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js" async></script> : null}
         </head>;
     }
 };

layout/common/scripts.jsx

@@ -31,12 +31,12 @@ module.exports = class extends Component {
         return <Fragment>
             <script src={cdn('jquery', '3.3.1', 'dist/jquery.min.js')}></script>
             <script src={cdn('moment', '2.22.2', 'min/moment-with-locales.min.js')}></script>
+            {clipboard && <script src={cdn('clipboard', '2.0.4', 'dist/clipboard.min.js')} async></script>}
             <script dangerouslySetInnerHTML={{ __html: `moment.locale("${language}");` }}></script>
             <script dangerouslySetInnerHTML={{ __html: embeddedConfig }}></script>
-            {clipboard ? <script src={cdn('clipboard', '2.0.4', 'dist/clipboard.min.js')} defer={true}></script> : null}
-            <script src={url_for('/js/column.js')}></script>
+            <script src={url_for('/js/column.js')} async></script>
             <Plugins site={site} config={config} page={page} helper={helper} head={false} />
-            <script src={url_for('/js/main.js')} defer={true}></script>
+            <script src={url_for('/js/main.js')} defer></script>
         </Fragment>;
     }
 };

layout/plugin/back_to_top.jsx

@@ -9,7 +9,7 @@ class BackToTop extends Component {
             <a id="back-to-top" title={title} href="javascript:;">
                 <i class="fas fa-chevron-up"></i>
             </a>
-            <script src={jsUrl} defer={true}></script>
+            <script src={jsUrl} defer></script>
         </Fragment>;
 
     }