[SPARK-7627] [SPARK-7472] DAG visualization: style skipped stages

This patch fixes two things:

**SPARK-7627.** Cached RDDs no longer light up on the job page. This is a simple fix.
**SPARK-7472.** Display skipped stages differently from normal stages.

The latter is a major UX issue. Because we link the job viz to the stage viz even for skipped stages, the user may inadvertently click into the stage page of a skipped stage, which is empty.

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<img src="https://cloud.githubusercontent.com/assets/2133137/7675241/de1a3da6-fcea-11e4-8101-88055cef78c5.png" width="300px" />

Author: Andrew Or <andrew@databricks.com>

Closes #6171 from andrewor14/dag-viz-skipped and squashes the following commits:

f261797 [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-skipped
0eda358 [Andrew Or] Tweak skipped stage border color
c604150 [Andrew Or] Tweak grayscale colors
7010676 [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-skipped
762b541 [Andrew Or] Use special prefix for stage clusters to avoid collisions
51c95b9 [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-skipped
b928cd4 [Andrew Or] Fix potential leak + write tests for it
7c4c364 [Andrew Or] Show skipped stages differently
7cc34ce [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-skipped
c121fa2 [Andrew Or] Fix cache color