Stack overflow Icon in Social Icons Block

### What

Add Stack Overflow icon to Social Icons block and widget

### Why

Stack Overflow is a popular social site for asking questions and providing answers to technical questions.

https://stackoverflow.com/

### How

Example of icon here
:

Start by locating the Social Icons block and widget entry points in the wp-calypso repository, then inspect how existing social icons are defined and tested. Add Stack Overflow support using the linked example as a visual reference, and verify that the icon appears in both the block and widget.

Written by the indexing model from the issue text.