[SPARK-2169] Don't copy appName / basePath everywhere.

Instead of keeping copies in all pages, just reference the values
kept in the base SparkUI instance (by making them available via
getters).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #1252 from vanzin/SPARK-2169 and squashes the following commits:

4412fc6 [Marcelo Vanzin] Simplify UIUtils.headerSparkPage signature.
4e5d35a [Marcelo Vanzin] [SPARK-2169] Don't copy appName / basePath everywhere.
[SPARK-2298] Encode stage attempt in SparkListener & UI.

Simple way to reproduce this in the UI:

```scala
val f = new java.io.File("/tmp/test")
f.delete()
sc.parallelize(1 to 2, 2).map(x => (x,x )).repartition(3).mapPartitionsWithContext { case (context, iter) =>
  if (context.partitionId == 0) {
    val f = new java.io.File("/tmp/test")
    if (!f.exists) {
      f.mkdir()
      System.exit(0);
    }
  }
  iter
}.count()
```

Author: Reynold Xin <rxin@apache.org>

Closes #1545 from rxin/stage-attempt and squashes the following commits:

3ee1d2a [Reynold Xin] - Rename attempt to retry in UI. - Properly report stage failure in FetchFailed.
40a6bd5 [Reynold Xin] Updated test suites.
c414c36 [Reynold Xin] Fixed the hanging in JobCancellationSuite.
b3e2eed [Reynold Xin] Oops previous code didn't compile.
0f36075 [Reynold Xin] Mark unknown stage attempt with id -1 and drop that in JobProgressListener.
6c08b07 [Reynold Xin] Addressed code review feedback.
4e5faa2 [Reynold Xin] [SPARK-2298] Encode stage attempt in SparkListener & UI.