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

This PR includes two changes
- **[SPARK-2147]** When an application finishes cleanly (i.e. `sc.stop()` is called), all of its executors used to disappear from the Master UI. This no longer happens.
- **[SPARK-2161]** This adds a "Removed Executors" table to Master UI, so the user can find out why their executors died from the logs, for instance. The equivalent table already existed in the Worker UI, but was hidden because of a bug (the comment `//scalastyle:off` disconnected the `Seq[Node]` that represents the HTML for table).

This should go into 1.0.1 if possible.
Mm, if we're going to use ExecutorInfo in a Set, we should at least implement equals() and hashCode() [and toString()?] in case it later actually comes into play. Both can probably just be based on the pre-existing fullId field. Or could make it a case class.