Upgrade Font Awesome to version 5 (#1446)

Upgrade Font Awesome to version 5 (SVG with JavaScript)

Close #1396

_sass/minimal-mistakes/_footer.scss

@@ -17,9 +17,9 @@
   margin-top: 3em;
   color: $muted-text-color;
   -webkit-animation: $intro-transition;
-          animation: $intro-transition;
+  animation: $intro-transition;
   -webkit-animation-delay: 0.45s;
-          animation-delay: 0.45s;
+  animation-delay: 0.45s;
   background-color: $footer-background-color;
 
   footer {
@@ -44,7 +44,10 @@
     }
   }
 
-  .fa {
+  .fas,
+  .fab,
+  .far,
+  .fal {
     color: $muted-text-color;
   }
 }
@@ -55,7 +58,6 @@
 }
 
 .page__footer-follow {
-
   ul {
     margin: 0;
     padding: 0;
@@ -80,4 +82,4 @@
     padding-right: 10px;
     font-weight: bold;
   }
-}
+}