[SPARK-2147 / 2161] Show removed executors on the UI

This PR includes two changes
- When an application finishes cleanly (i.e. `sc.stop()` is called), all of its executors disappear from the Master UI. It might be useful to keep them around so the user can click on the executor logs even after the application finishes.
- This adds a "Removed Executors" table to Master UI, so the user can find out why their executors died, for instance. The equivalent table already exists in the Worker UI, but was hidden because of a bug. (The bug is that the comment `//scalastyle:off` disconnected the `Seq[Node]` that represents the HTML for table)