[SPARK-14576][WEB UI] Spark console should display Web UI url

## What changes were proposed in this pull request?
This is a proposal to print the Spark Driver UI link when spark-shell is launched.

## How was this patch tested?
Launched spark-shell in local mode and cluster mode. Spark-shell console output included following line:
"Spark context Web UI available at <Spark web url>"

Author: Ergin Seyfe <eseyfe@fb.com>

Closes #12341 from seyfe/spark_console_display_webui_link.