Bug 1220680 - Make detail highlighting more consistent / subtle in compare view

* low/medium/high confidence explanation now uses same tooltip as the other
  elements
* we now only underline the currently hovered row

ui/css/perf.css

@@ -263,6 +263,10 @@ to { opacity: 0; }
     text-align: left;
 }
 
+.compare-table tr:hover .detail-hint {
+    border-bottom: 1px dotted #777;
+}
+
 .compare-table .num-runs {
     text-align: right;
 }
@@ -313,7 +317,3 @@ span.compare-regression {
 .bar-regression {
   background-color: red;
 }
-
-.detail-hint {
-    border-bottom: 1px dotted #777;
-}

ui/partials/perf/comparetable.html

@@ -68,7 +68,7 @@
       <td ng-if="!compareResult.delta"></td>
       <td>
         <span ng-if="compareResult.confidenceText && compareResult.confidence">
-          {{compareResult.confidence|displayPrecision}} (<abbr title="Result of running t-test on base versus new result distribution: higher values indicate greater confidence that the indicated performance difference is statistically meaningful">{{compareResult.confidenceText}}</abbr>)</span>&nbsp;
+          {{compareResult.confidence|displayPrecision}} (<span class="detail-hint" tooltip="Result of running t-test on base versus new result distribution: higher values indicate greater confidence that the indicated performance difference is statistically meaningful">{{compareResult.confidenceText}}</abbr>)</span>&nbsp;
       </td>
       <td class="num-runs">
         <span class="detail-hint"