Allow Google Analytics to use gtag.js (#601)

* Moved google analytics script to head

The code now uses the new gtag script to track the website on google analytics.
The migration is explained [here](https://developers.google.com/analytics/devguides/collection/gtagjs/migration).
This will help to solve issues such as #375.
As indicated at https://support.google.com/analytics/answer/1008080
the analytics code must be placed in the head tag, so it is moved there.

* Add a boolean to use the new gtag library

* Update api-site-config.md

lib/core/Site.js

@@ -83,21 +83,6 @@ class Site extends React.Component {
               src="https://cdn.jsdelivr.net/docsearch.js/1/docsearch.min.js"
             />
           )}
-          {this.props.config.gaTrackingId && (
-            <script
-              dangerouslySetInnerHTML={{
-                __html: `
-              (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
-              (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
-              m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
-              })(window,document,'script','https://www.google-analytics.com/analytics.js','ga');
-
-              ga('create', '${this.props.config.gaTrackingId}', 'auto');
-              ga('send', 'pageview');
-            `,
-              }}
-            />
-          )}
           {this.props.config.facebookAppId && (
             <script
               dangerouslySetInnerHTML={{