[SPARK-21060][WEB-UI] Css style about paging function is error in the…

… executor page. Css style about paging function is error in the executor page. It is different of history server ui paging function css style.

## What changes were proposed in this pull request?

Css style about paging function is error in the executor page. It is different of history server ui paging function css style.

**But their style should be consistent**. There are three reasons.

1. The first reason: 'Previous', 'Next' and number should be the button format.

2. The second reason: when you are on the first page, 'Previous' and '1' should be gray and can not be clicked.
![1](https://user-images.githubusercontent.com/26266482/27026667-1fe745ee-4f91-11e7-8b34-150819d22bd3.png)

3. The third reason: when you are on the last page, 'Previous' and 'Max number' should be gray and can not be clicked.
![2](https://user-images.githubusercontent.com/26266482/27026811-9d8d6fa0-4f91-11e7-8b51-7816c3feb381.png)

before fix:
![fix_before](https://user-images.githubusercontent.com/26266482/27026428-47ec5c56-4f90-11e7-9dd5-d52c22d7bd36.png)

after fix:
![fix_after](https://user-images.githubusercontent.com/26266482/27026439-50d17072-4f90-11e7-8405-6f81da5ab32c.png)

The style of history server ui:
![history](https://user-images.githubusercontent.com/26266482/27026528-9c90f780-4f90-11e7-91e6-90d32651fe03.png)

## How was this patch tested?

manual tests

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: guoxiaolong <guo.xiaolong1@zte.com.cn>
Author: 郭小龙 10207633 <guo.xiaolong1@zte.com.cn>
Author: guoxiaolongzte <guo.xiaolong1@zte.com.cn>

Closes apache#18275 from guoxiaolongzte/SPARK-21060.

core/src/main/scala/org/apache/spark/ui/exec/ExecutorsPage.scala

@@ -82,7 +82,7 @@ private[ui] class ExecutorsPage(
               </ul>
             </div>
           </div> ++
-          <div id="active-executors"></div> ++
+          <div id="active-executors" class="span12 pagination"></div> ++
           <script src={UIUtils.prependBaseUri("/static/utils.js")}></script> ++
           <script src={UIUtils.prependBaseUri("/static/executorspage.js")}></script> ++
           <script>setThreadDumpEnabled({threadDumpEnabled})</script>