[SPARK-1301][WEB UI] Added anchor links to Accumulators and Tasks on StagePage

## What changes were proposed in this pull request?

Sometimes the "Aggregated Metrics by Executor" table on the Stage page can get very long so actor links to the Accumulators and Tasks tables below it have been added to the summary at the top of the page. This has been done in the same way as the Jobs and Stages pages. Note: the Accumulators link only displays when the table exists.

## How was this patch tested?

Manually Tested and dev/run-tests

![justtasks](https://cloud.githubusercontent.com/assets/13952758/15165269/6e8efe8c-16c9-11e6-9784-cffe966fdcf0.png)
![withaccumulators](https://cloud.githubusercontent.com/assets/13952758/15165270/7019ec9e-16c9-11e6-8649-db69ed7a317d.png)

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

Closes #13037 from ajbozarth/spark1301.