[SPARK-4141] Hide Accumulators column on stage page when no accumulators exist

WebUI

Author: Mark Mims <mark.mims@canonical.com>

This patch had conflicts when merged, resolved by
Committer: Josh Rosen <joshrosen@databricks.com>

Closes #3031 from mmm/remove-accumulators-col and squashes the following commits:

6141cb3 [Mark Mims] reformat to satisfy scalastyle linelength.  build failed from jenkins https://amplab.cs.berkeley.edu/jenkins//job/SparkPullRequestBuilder/22604/
390893b [Mark Mims] cleanup
c28c449 [Mark Mims] looking much better now... minimal explicit formatting.  Now, see if any sort keys make sense
fb72156 [Mark Mims] mimic hasInput.  The basics work here, but wanna clean this up with maybeAccumulators for column content