ENH: Add user feedback section in Welcome module.

Added an offer for users to share their stories and feedback using the
google form created by Steve Pieper. See http://goo.gl/6BvcHm

Fixes #3229

Co-authored-by: Jean-Christophe Fillion-Robin <JChris.FillionR@kitware.com>

From: Yuzheng Zhou <yuzheng.zhou@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22475 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix conversion-null warnings in qSlicerExtensionsManagerModel

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22476 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Remove unused includes in qSlicerWebWidget

From: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22477 3bd1e089-480b-0410-8dfb-8563597acbee