[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)

I think you need to move this to before the initialize() call -- as is, kill is always disabled, because killEnabled is false when initialize is called, so StagesTab will be initialized with killEnabled set to false (I noticed this when I was playing around with this, because the kill button was nowhere to be found).