[SPARK-7664] [WEBUI] DAG visualization: Fix incorrect link paths of DAG.

In JobPage, we can jump a StagePage when we click corresponding box of DAG viz but the link path is incorrect.

When we click a box like as follows ...
![screenshot_from_2015-05-15 19 24 25](https://cloud.githubusercontent.com/assets/4736016/7651528/5f7ef824-fb3c-11e4-9518-8c9ade2dff7a.png)

We jump to index page.
![screenshot_from_2015-05-15 19 24 45](https://cloud.githubusercontent.com/assets/4736016/7651534/6d666274-fb3c-11e4-971c-c3f2dc2b1da2.png)

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #6184 from sarutak/fix-link-path-of-dag-viz and squashes the following commits:

faba3ba [Kousuke Saruta] Fix a incorrect link