[SPARK-7371] [SPARK-7377] [SPARK-7408] DAG visualization addendum (#5729)

This is a follow-up patch for #5729.

**[SPARK-7408]** Move as much style code from JS to CSS as possible
**[SPARK-7377]** Fix JS error if a job / stage contains only one RDD
**[SPARK-7371]** Decrease emphasis on RDD on stage page as requested by mateiz pwendell

This patch also includes general code clean up.

<img src="https://issues.apache.org/jira/secure/attachment/12730992/before-after.png" width="500px"></img>

Author: Andrew Or <andrew@databricks.com>

Closes #5954 from andrewor14/viz-emphasize-rdd and squashes the following commits:

3c0d4f0 [Andrew Or] Guard against JS error by rendering arrows only if needed
f23e15b [Andrew Or] Merge branch 'master' of github.com:apache/spark into viz-emphasize-rdd
565801f [Andrew Or] Clean up code
9dab5f0 [Andrew Or] Move styling from JS to CSS + clean up code
107c0b6 [Andrew Or] Tweak background color, stroke width, font size etc.
1610c62 [Andrew Or] Implement cluster padding for stage page