[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 <[email protected]>

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

Author: Andrew Or

core/src/main/resources/org/apache/spark/ui/static/dagre-d3.min.js

@@ -0,0 +1,83 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#dag-viz-graph svg path {
+  stroke: #444444;
+  stroke-width: 1.5px;
+}
+
+#dag-viz-graph svg g.cluster rect {
+  stroke-width: 4px;
+  stroke-opacity: 0.5;
+}
+
+#dag-viz-graph svg g.node circle,
+#dag-viz-graph svg g.node rect {
+  fill: #444444;
+}
+
+#dag-viz-graph svg g.node.cached circle,
+#dag-viz-graph svg g.node.cached rect {
+  fill: #FF0000;
+}
+
+/* Job page specific styles */
+
+#dag-viz-graph svg.job marker#marker-arrow path {
+  fill: #444444;
+  stroke-width: 0px;
+}
+
+#dag-viz-graph svg.job g.cluster rect {
+  fill: #FFFFFF;
+  stroke: #AADFFF;
+}
+
+#dag-viz-graph svg.job g.cluster[id*="stage"] rect {
+  stroke: #FFDDEE;
+  stroke-width: 6px;
+}
+
+#dag-viz-graph svg.job g#cross-stage-edges path {
+  fill: none;
+}
+
+#dag-viz-graph svg.job g.cluster text {
+  fill: #AAAAAA;
+}
+
+/* Stage page specific styles */
+
+#dag-viz-graph svg.stage g.cluster rect {
+  fill: #F0F8FF;
+  stroke: #AADFFF;
+}
+
+#dag-viz-graph svg.stage g.cluster[id*="stage"] rect {
+  fill: #FFFFFF;
+  stroke: #FFDDEE;
+  stroke-width: 6px;
+}
+
+#dag-viz-graph svg.stage g.node g.label text tspan {
+  fill: #FFFFFF;
+}
+
+#dag-viz-graph svg.stage g.cluster text {
+  fill: #444444;
+  font-weight: bold;
+}