docs: convert website from postcss to emotion (#2068)

website/src/html.js

@@ -1,4 +1,5 @@
 import React from 'react';
+import ChatButton from './components/chat-button';
 
 class HTML extends React.Component {
   render() {
@@ -24,9 +25,7 @@ class HTML extends React.Component {
           {this.props.preBodyComponents}
           <div key={'body'} id="___gatsby" dangerouslySetInnerHTML={{ __html: this.props.body }} />
           {this.props.postBodyComponents}
-          <a className="chat-button" href="/chat">
-            <img src="/img/slack.svg" />
-          </a>
+          <ChatButton />
           <script src="//unpkg.com/docsearch.js@2.4.1/dist/cdn/docsearch.min.js" />
         </body>
       </html>