Two minor bug fixes for Spark Web UI

This pull request fixes two minor issues in the Spark Web UI:

- [SPARK-630](https://spark-project.atlassian.net/browse/SPARK-630): App details page lists finished/killed executors as RUNNING.
- [SPARK-629](https://spark-project.atlassian.net/browse/SPARK-629): App details page has strange value for number of cores.

I also fixed a minor HTML issue where an `<a>` tag was closed with `</href>`.