[SPARK-8145] [WEBUI] Trigger a double click on the span to show full job description.

When using the Spark SQL, Jobs tab and Stages tab display only part of SQL. I change it to  display full SQL by double-click on the description span

before：
![before](https://cloud.githubusercontent.com/assets/5399861/8022257/9f8e0a22-0cf8-11e5-98c8-da4d7a615e7e.png)

after double click on the description span：
![after](https://cloud.githubusercontent.com/assets/5399861/8022261/dac08d4a-0cf8-11e5-8fe7-74c96c6ce933.png)

Author: 979969786 <q79969786@gmail.com>

Closes #6646 from 979969786/master and squashes the following commits:

b5ba20e [979969786] Trigger a double click on the span to show full job description.

core/src/main/resources/org/apache/spark/ui/static/additional-metrics.js

@@ -50,4 +50,9 @@ $(function() {
     $("span.additional-metric-title").click(function() {
         $(this).parent().find('input[type="checkbox"]').trigger('click');
     });
+
+    // Trigger a double click on the span to show full job description.
+    $(".description-input").dblclick(function() {
+        $(this).removeClass("description-input").addClass("description-input-full");
+    });
 });

core/src/main/resources/org/apache/spark/ui/static/webui.css

@@ -135,6 +135,14 @@ pre {
   display: block;
 }
 
+.description-input-full {
+  overflow: hidden;
+  text-overflow: ellipsis;
+  width: 100%;
+  white-space: normal;
+  display: block;
+}
+
 .stacktrace-details {
   max-height: 300px;
   overflow-y: auto;