Fix font icon line-height

We forgot to include this property when replacing our use of `%fa-icon`,
and it was causing the admin menu to have a blank space at the bottom.

So we're including it again to make sure nothing else breaks because of
this omition.

app/assets/stylesheets/mixins/icons.scss

@@ -1,5 +1,5 @@
 %font-icon {
-  display: inline-block;
+  @extend %fa-icon;
   font-family: "Font Awesome 5 Free";
   vertical-align: middle;
 }