Add a button to download thread dump as a txt file in Spark UI

Add a new button on the thread dump page of Spark Web Ui to download the thread dump in text format.

Right now if a user wants to collect a thread dump for an executor or driver using Spark UI, the thread dump is displayed as an html page. User can click on expand all button and then save that html file or take screen shots. However for debugging purpose it is always helpful to have the jstack output in raw text format without any html formatting and the process of saving the file should be simple as one click. In this Jira we propose to add a button on the Spark Ui page to be able to directly save or display the thread dump output in raw text format without html formatting.