<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.8.1/css/all.css" integrity="sha384-50oBUHEmvpQ+1lW4y57PTFmhCaXp0ML5d60M1M7uH2+nqUivzIebhndOJK28anvf" crossorigin="anonymous">
<i class="fas fa-user-astronaut"></i>
https:
и оставить только слэши.//use.fontawesome.com/releases/v5.8.1/css/all.css