[SPARK-2411] Add a history-not-found page to standalone Master

Right now, if you click on an application after it has finished, it simply refreshes the page if there are no event logs for the application. This is not super intuitive especially because event logging is not enabled by default. We should direct the user to enable this if they attempt to view a SparkUI after the fact without event logs.

Here is what the new page looks like:

<img src="https://issues.apache.org/jira/secure/attachment/12655082/Master%20event%20logs.png">
This is for the standalone Master, not the history server. Here the event log directory for each application is shipped to the master as part of the application description, so the Master knows where to find the logs automatically.

(For history server, however, the user should explicitly set the path in all cases as you describe)