[SPARK-23470][UI] Use first attempt of last stage to define job description.

This is much faster than finding out what the last attempt is, and the
data should be the same.

There's room for improvement in this page (like only loading data for
the jobs being shown, instead of loading all available jobs and sorting
them), but this should bring performance on par with the 2.2 version.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #20644 from vanzin/SPARK-23470.