[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`.

### 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/50748/5134910/50b340d4-70c7-11e4-88e1-6b73237ea7c8.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.

Author: Josh Rosen <joshrosen@databricks.com>

Closes #3009 from JoshRosen/job-page and squashes the following commits:

eb05e90 [Josh Rosen] Disable kill button in completed stages tables.
f00c851 [Josh Rosen] Fix JsonProtocol compatibility
b89c258 [Josh Rosen] More JSON protocol backwards-compatibility fixes.
ff804cd [Josh Rosen] Don't write "Stage Ids" field in JobStartEvent JSON.
6f17f3f [Josh Rosen] Only store StageInfos in SparkListenerJobStart event.
2bbf41a [Josh Rosen] Update job progress bar to reflect skipped tasks/stages.
61c265a [Josh Rosen] Add “skipped stages” table; only display non-empty tables.
1f45d44 [Josh Rosen] Incorporate a bunch of minor review feedback.
0b77e3e [Josh Rosen] More bug fixes for phantom stages.
034aa8d [Josh Rosen] Use `.max()` to find result stage for job.
eebdc2c [Josh Rosen] Don’t display pending stages for completed jobs.
67080ba [Josh Rosen] Ensure that "phantom stages" don't cause memory leaks.
7d10b97 [Josh Rosen] Merge remote-tracking branch 'apache/master' into job-page
d69c775 [Josh Rosen] Fix table sorting on all jobs page.
5eb39dc [Josh Rosen] Add pending stages table to job page.
f2a15da [Josh Rosen] Add status field to job details page.
171b53c [Josh Rosen] Move `startTime` to the start of SparkContext.
e2f2c43 [Josh Rosen] Fix sorting of stages in job details page.
8955f4c [Josh Rosen] Display information for pending stages on jobs page.
8ab6c28 [Josh Rosen] Compute numTasks from job start stage infos.
5884f91 [Josh Rosen] Add StageInfos to SparkListenerJobStart event.
79793cd [Josh Rosen] Track indices of completed stage to avoid overcounting when failures occur.
d62ea7b [Josh Rosen] Add failing Selenium test for stage overcounting issue.
1145c60 [Josh Rosen] Display text instead of progress bar for stages.
3d0a007 [Josh Rosen] Merge remote-tracking branch 'origin/master' into job-page
8a2351b [Josh Rosen] Add help tooltip to Spark Jobs page.
b7bf30e [Josh Rosen] Add stages progress bar; fix bug where active stages show as completed.
4846ce4 [Josh Rosen] Hide "(Job Group") if no jobs were submitted in job groups.
4d58e55 [Josh Rosen] Change label to "Tasks (for all stages)"
85e9c85 [Josh Rosen] Extract startTime into separate variable.
1cf4987 [Josh Rosen] Fix broken kill links; add Selenium test to avoid future regressions.
56701fa [Josh Rosen] Move last stage name / description logic out of markup.
a475ea1 [Josh Rosen] Add progress bars to jobs page.
45343b8 [Josh Rosen] More comments
4b206fb [Josh Rosen] Merge remote-tracking branch 'origin/master' into job-page
bfce2b9 [Josh Rosen] Address review comments, except for progress bar.
4487dcb [Josh Rosen] [SPARK-4145] Web UI job pages
2568a6c [Josh Rosen] Rename JobProgressPage to AllStagesPage: