[SPARK-15083][WEB UI] History Server can OOM due to unlimited TaskUIData

## What changes were proposed in this pull request?

Based on #12990 by tankkyo

Since the History Server currently loads all application's data it can OOM if too many applications have a significant task count. `spark.ui.trimTasks` (default: false) can be set to true to trim tasks by `spark.ui.retainedTasks` (default: 10000)

(This is a "quick fix" to help those running into the problem until a update of how the history server loads app data can be done)

## How was this patch tested?

Manual testing and dev/run-tests

![spark-15083](https://cloud.githubusercontent.com/assets/13952758/17713694/fe82d246-63b0-11e6-9697-b87ea75ff4ef.png)

Author: Alex Bozarth <ajbozart@us.ibm.com>

Closes #14673 from ajbozarth/spark15083.