SPARK-2250: show stage RDDs in UI

Author: Neville Li <neville@spotify.com>

Closes #1188 from nevillelyh/neville/ui and squashes the following commits:

d3ac425 [Neville Li] SPARK-2250: show persisted RDD in stage UI
f075db9 [Neville Li] SPARK-2035: show call stack even when description is available