Show banner/full page for the Ditigal Climate Strike

As discussed in the mailing list, pytest will join the
digital strike for the climate on Sep 20th. This will show
a closable banner on the docs until the date, and when the date
comes the banner will become a (closable) full page.

I will also pin an issue saying that the developers won't be
available on Sep 20th.

doc/en/_templates/layout.html

@@ -17,4 +17,46 @@
   })();
 
 </script>
+<script type="text/javascript">
+  var DIGITAL_CLIMATE_STRIKE_OPTIONS = {
+
+    /**
+     * Allow you to override the iFrame hostname. Defaults to https://assets.digitalclimatestrike.net
+     */
+    iframeHost: 'https://assets.digitalclimatestrike.net', // @type {string}
+
+    /**
+     * Prevents the widget iframe from loading Google Analytics. Defaults to
+     * false. (Google Analytics will also be disabled if doNotTrack is set on
+     * the user's browser.)
+     */
+    disableGoogleAnalytics: false, // @type {boolean}
+
+    /**
+     * Always show the widget, even when someone has closed the widget and set the cookie on their device.
+     * Useful for testing. Defaults to false.
+     */
+    alwaysShowWidget: false, // @type {boolean}
+
+    /**
+     * Automatically makes the widget full page. Defaults to false.
+     */
+    forceFullPageWidget: false, // @type {boolean}
+
+    /**
+    * For the full page widget, shows a close button "x" and hides the message about the site being
+    * available tomorrow. Defaults to false.
+    */
+    showCloseButtonOnFullPageWidget: true, // @type {boolean}
+
+
+    /**
+     * The date when the full page widget should showing on your web site for 24 hours.
+     * Note: the month is one integer less than the number of the month. E.g. 8 is September, not August.
+     * Defaults to new Date(2019, 8, 20) (September 20th, 2019)
+     */
+    fullPageDisplayStartDate: new Date(2019, 8, 20), //@ type {Date object}
+  };
+</script>
+<script src="https://assets.digitalclimatestrike.net/widget.js" async></script>
 {% endblock %}