[SPARK-4145] Web UI job pages

This PR adds two new pages to the Spark Web UI:

- A jobs overview page, which shows details on running / completed / failed jobs.
- A job details page, which displays information on an individual job's stages.

The jobs overview page is now the default UI homepage; the old homepage is still accessible at /stages.

In some situations, the jobs page may display "No information on stage" for some stages of pending jobs that have not begun executing.  This is due to some limitations in how JobProgressListener finds out about stages.  We can address this later as part of a separate scheduler PR.

### Screenshots

#### New UI homepage

![image](https://cloud.githubusercontent.com/assets/50748/4989750/801ff1ce-6932-11e4-8d1a-1d2a3c527227.png)


#### Job details page

(This is effectively a per-job version of the stages page that can be extended later with other things, such as DAG visualizations)

![image](https://cloud.githubusercontent.com/assets/50748/4837239/79ee6b1e-5fd2-11e4-839d-6cbb42fb01c0.png)

This is a good catch.  It turns out that this was a pretty serious mistake, since the `parent.killEnabled` that i'm passing here was actually being used for the `isFairScheduler` boolean.  I've explicitly named my boolean parameters to avoid this sort of mistake.  I've also added a Selenium test to catch these sorts of regressions where `spark.ui.killEnabled` isn't respected.