Content styles: Made links underlined for visibility

Inline with A11y recommendations where color may not be reliable on its
own.
Tested various content link scenarios across chrome, safari & FF.
For #4939

resources/sass/_content.scss

@@ -113,6 +113,10 @@
   video {
     max-width: 100%;
   }
+
+  a {
+    text-decoration: underline;
+  }
 }
 
 /**