[SPARK-23475][UI] Show also skipped stages

## What changes were proposed in this pull request?

SPARK-20648 introduced the status `SKIPPED` for the stages. On the UI, previously, skipped stages were shown as `PENDING`; after this change, they are not shown on the UI.

The PR introduce a new section in order to show also `SKIPPED` stages in a proper table.

## How was this patch tested?

manual tests

Author: Marco Gaido <marcogaido91@gmail.com>

Closes #20651 from mgaido91/SPARK-23475.