Clean up code

In particular, the code that draws the cluster labels now takes
into account of the specific font-size of the label. Also, less
important functions are now grouped to the end to make it clear
that they should not be given as much attention. This makes the
code easier to follow.

Author: Andrew Or

core/src/main/resources/org/apache/spark/ui/static/spark-dag-viz.css

@@ -58,7 +58,6 @@
 
 #dag-viz-graph svg.job g.cluster text {
   fill: #AAAAAA;
-  font-size: 11px;
 }
 
 /* Stage page specific styles */
@@ -80,6 +79,5 @@
 
 #dag-viz-graph svg.stage g.cluster text {
   fill: #444444;
-  font-size: 14px;
   font-weight: bold;
 }