Clean up web UI page headers

This removes the master URL from the job UI and reduces the heading size of basic Spark pages, fixing [SPARK-828](https://spark-project.atlassian.net/browse/SPARK-828).
![navbar](https://f.cloud.github.com/assets/4754931/888936/1c70467e-fa16-11e2-9e43-b9479c168f2d.png)
![heading after](https://f.cloud.github.com/assets/4754931/888938/1c9c4b5c-fa16-11e2-87fc-d0bcc2306f0d.png)