[SPARK-2299] Consolidate various stageIdTo* hash maps in JobProgressListener

This should reduce memory usage for the web ui as well as slightly increase its speed in draining the UI event queue.

@andrewor14

Author: Reynold Xin <rxin@apache.org>

Closes #1262 from rxin/ui-consolidate-hashtables and squashes the following commits:

1ac3f97 [Reynold Xin] Oops. Properly handle description.
f5736ad [Reynold Xin] Code review comments.
b8828dc [Reynold Xin] Merge branch 'master' into ui-consolidate-hashtables
7a7b6c4 [Reynold Xin] Revert css change.
f959bb8 [Reynold Xin] [SPARK-2299] Consolidate various stageIdTo* hash maps in JobProgressListener to speed it up.
63256f5 [Reynold Xin] [SPARK-2320] Reduce <pre> block font size.