[SPARK-7864] [UI] Do not kill innocent stages from visualization

**Reproduction.** Run a long-running job, go to the job page, expand the DAG visualization, and click into a stage. Your stage is now killed. Why? This is because the visualization code just reaches into the stage table and grabs the first link it finds. In our case, this first link happens to be the kill link instead of the one to the stage page.

**Fix.** Use proper CSS selectors to avoid ambiguity.

This is an alternative to #6407. Thanks carsonwang for catching this.

Author: Andrew Or <andrew@databricks.com>

Closes #6419 from andrewor14/fix-ui-viz-kill and squashes the following commits:

25203bd [Andrew Or] Do not kill innocent stages