[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 <[email protected]>
Author: 郭小龙 10207633 <[email protected]>
Author: guoxiaolongzte <[email protected]>

Closes apache#18275 from guoxiaolongzte/SPARK-21060.

Author: guoxiaolong

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>