Fix hover colors on buttons to show/hide content

We were setting the color to be the same as links, but forgot to do the
same on hover. Using the `link` mixin solves the issue.

app/assets/stylesheets/advanced_search.scss

@@ -29,8 +29,7 @@
 
   > [aria-expanded] {
     @include xy-gutters;
-    color: $link;
-    cursor: pointer;
+    @include link;
     margin-top: $line-height;
     margin-bottom: $line-height;
     max-width: max-content;

app/assets/stylesheets/layout.scss

@@ -2359,8 +2359,7 @@ table {
 }
 
 .add-related-content {
-  color: $link;
-  cursor: pointer;
+  @include link;
   display: block;
   margin-bottom: $line-height;