Use a Font Awesome icon in recommendations

We've deprecated the "icons" font since we started using Font Awesome
two years ago and using it caused some screen readers to announce an "l"
before the content of every list item.

app/assets/stylesheets/participation.scss

@@ -250,11 +250,11 @@
   margin-top: $line-height;
 
   li {
+    @include has-fa-icon(check, solid);
     margin: $line-height / 2 0;
 
     &::before {
-      content: "l ";
-      font-family: "icons" !important;
+      margin-right: $font-icon-margin * 1.5;
     }
   }
 }