Add a scroll to top button siteConfig option (#723)

lib/core/Head.js

@@ -149,6 +149,33 @@ class Head extends React.Component {
             );
           })}
 
+        {this.props.config.scrollToTop && (
+          <script
+            src={
+              'https://unpkg.com/vanilla-back-to-top@7.1.14/dist/vanilla-back-to-top.min.js'
+            }
+          />
+        )}
+        {this.props.config.scrollToTop && (
+          <script
+            dangerouslySetInnerHTML={{
+              __html: `
+        document.addEventListener("DOMContentLoaded", function(){
+          addBackToTop(
+            ${JSON.stringify(
+              Object.assign(
+                {},
+                {zIndex: 100},
+                this.props.config.scrollToTopOptions
+              )
+            )}
+          )
+        });
+        `,
+            }}
+          />
+        )}
+
         {/* Site defined code. Keep these at the end to avoid overriding. */}
         <link
           rel="stylesheet"