Fix #3976: DText external links appear broken.

app/javascript/src/styles/base/000_vars.scss

@@ -53,14 +53,14 @@ $success_color: hsl(120, 100%, 95%); // light green
 }
 
 // https://fontawesome.com/how-to-use/on-the-web/advanced/css-pseudo-elements
-@mixin fa-icon($family, $weight, $content) {
+@mixin fa-solid-icon($content) {
   display: inline-block;
   font-style: normal;
   font-variant: normal;
   text-rendering: auto;
   -webkit-font-smoothing: antialiased;
 
-  font-family: $family;
-  font-weight: $weight;
+  font-family: "Font Awesome 5 Free", "Font Awesome 5 Pro";
+  font-weight: 900;
   content: $content;
 }

app/javascript/src/styles/common/dtext.scss

@@ -90,7 +90,7 @@ div.prose {
 
   a.dtext-external-link::after {
     // https://fontawesome.com/icons/external-link-alt?style=solid
-    @include fa-icon("Font Awesome 5 Free", 900, "\f35d");
+    @include fa-solid-icon("\f35d");
 
     font-size: 0.6em;
     padding: 0 0.2em 0 0.3em;