[SPARK-4145] [WIP] 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`.

### Screenshots

#### New UI homepage

![image](https://cloud.githubusercontent.com/assets/50748/5119035/fd0a69e6-701f-11e4-89cb-db7e9705714f.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/50748/5119053/52ae737e-7020-11e4-9fe3-6aab694609ec.png)

### Key changes in this PR

- Rename `JobProgressPage` to `AllStagesPage`
- Expose `StageInfo` objects in the ``SparkListenerJobStart` event; add backwards-compatibility tests to JsonProtocol.
- Add additional data structures to `JobProgressListener` to map from stages to jobs.
- Add several fields to `JobUIData`.

I also added ~150 lines of Selenium tests as I uncovered UI issues while developing this patch.

### Limitations

If a job contains stages that aren't run, then its overall job progress bar may be an underestimate of the total job progress; in other words, a completed job may appear to have a progress bar that's not at 100%.

If stages or tasks fail, then the progress bar will not go backwards to reflect the true amount of remaining work.
I'd be happy to chat offline about this, but I'm concerned that a `KillableStagesTable` might be very difficult to implement in our current UI framework because we don't have a great way to subclass / override portions of the HTML generation; there's not really any modularization / interfaces / etc. in any of our markup generation.

I played around with some ideas to improve this over at #2852 but eventually abandoned that approach because I ran into corner-cases that my abstraction couldn't cleanly handle.