[SPARK-4313][WebUI][Yarn] Fix link issue of the executor thread dump page in yarn-cluster mode

In yarn-cluster mode, the Web UI is running behind a yarn proxy server. Some features(or bugs?) of yarn proxy server will break the links for thread dump.

1. Yarn proxy server will do http redirect internally, so if opening `http://example.com:8088/cluster/app/application_1415344371838_0012/executors`, it will fetch `http://example.com:8088/cluster/app/application_1415344371838_0012/executors/` and return the content but won't change the link in the browser. Then when a user clicks `Thread Dump`, it will jump to `http://example.com:8088/proxy/application_1415344371838_0012/threadDump/?executorId=2`. This is a wrong link. The correct link should be `http://example.com:8088/proxy/application_1415344371838_0012/executors/threadDump/?executorId=2`.

Adding "/" to the tab links will fix it.

2. Yarn proxy server has a bug about the URL encode/decode. When a user accesses `http://example.com:8088/proxy/application_1415344371838_0006/executors/threadDump/?executorId=%3Cdriver%3E`, the yarn proxy server will require `http://example.com:36429/executors/threadDump/?executorId=%25253Cdriver%25253E`. But Spark web server expects `http://example.com:36429/executors/threadDump/?executorId=%3Cdriver%3E`. Related to [YARN-2844](https://issues.apache.org/jira/browse/YARN-2844).

For now, it's a tricky approach to bypass the yarn bug.

![threaddump](https://cloud.githubusercontent.com/assets/1000778/4972567/d1ccba64-68ad-11e4-983e-257530cef35a.png)