Use js-hover instead of card-emphasis

h/static/styles/threads.scss

@@ -7,7 +7,7 @@ $threadexp-width: .6em;
   }
 
   & > li.thread:hover,
-  .card-emphasis {
+  & > li.thread.js-hover {
     @include focus-outline;
   }
 }

h/templates/client/viewer.html

@@ -34,7 +34,7 @@
     </span>
   </li>
   <li class="paper thread"
-      ng-class="{'card-emphasis': isFocused(child.message)}"
+      ng-class="{'js-hover': isFocused(child.message)}"
       deep-count="count"
       thread="child" thread-filter
       thread-collapsed="!search.query"