[SPARK-15885][WEB UI] Provide links to executor logs from stage details page in UI

## What changes were proposed in this pull request?

This moves over old PR https://github.com/apache/spark/pull/13664 to target master rather than branch-1.6.

Added links to logs (or an indication that there are no logs) for entries which list an executor in the stage details page of the UI.

This helps streamline the workflow where a user views a stage details page and determines that they would like to see the associated executor log for further examination.  Previously, a user would have to cross reference the executor id listed on the stage details page with the corresponding entry on the executors tab.

Link to the JIRA: https://issues.apache.org/jira/browse/SPARK-15885

## How was this patch tested?

Ran existing unit tests.
Ran test queries on a platform which did not record executor logs and again on a platform which did record executor logs and verified that the new table column was empty and links to the logs (which were verified as linking to the appropriate files), respectively.

Attached is a screenshot of the UI page with no links, with the new columns highlighted.  Additional screenshot of these columns with the populated links.

Without links:
![updated without logs](https://cloud.githubusercontent.com/assets/1450821/16059721/2b69dbaa-3239-11e6-9eed-e539764ca159.png)

With links:
![updated with logs](https://cloud.githubusercontent.com/assets/1450821/16059725/32c6e316-3239-11e6-90bd-2553f43f7779.png)

This contribution is my original work and I license the work to the project under the Apache Spark project's open source license.

Author: Tom Magrino <tmagrino@fb.com>

Closes #13861 from tmagrino/uilogstweak.