Use WP style for intro.js buttons & make nice prev-next/prev-done button group

The intention here is to use WP default styles (adding .button class to buttons)
with as little overrides as possible, avoiding to use absolute px sizes.

[MAILPOET-1446]

assets/js/src/intro.jsx

@@ -29,6 +29,9 @@ function Intro() {
     skipLabel: 'Skip',
     doneLabel: 'Done',
     positionPrecedence: ['right', 'left', 'bottom', 'top'],
+    buttonClass: 'button',
+    hidePrev: true,
+    hideNext: true,
     helperElementPadding: 12,
     scrollToElement: false,
     showStepNumbers: false,