Add styles for a tags in spoiler

Links were showing through the spoiler due to their text color. Add
style to make their text color black.

static/css/components.css

@@ -38,6 +38,9 @@
     .spoiler:not(:hover) * {
         background-color: inherit;
     }
+    .spoiler a {
+        @apply text-black;
+    }
 
     /* Styles for modals */
     .modal-overlay {