News Article Featured Image add image from site library (re-use Add Media button functionality for choosing featured image)

Issue Summary

Problem/Motivation

(why the issue was filed, steps to reproduce the problem, etc.)
The "choose file" button for the "Featured Image" only provides file upload functionality for uploading an image file from local machine only. By contrast, the "add media" button on the WYSIWYG tool bar not only provides upload from local machine but also the ability to use an image already in the site (select the "library" tab to browse such images).

Proposed resolution

(description of the proposed solution, the rationale behind it, and workarounds for people who cannot use the patch)
Re-use/copy the code that is triggered by the "Add Media" button on the WYSIWYG tool bar to provide a chooser to browse the site images ("library").

Remaining tasks

(reviews needed, tests to be written or run, documentation to be written, etc.)
To start once an implementation has been provided

User interface changes

(new or changed features/functionality in the user interface, modules added or removed, changes to URL paths, changes to user interface text)
Change the resultant outcome of pressing the "choose file" button for the "Featured Image" in a Panopoly News Article.

API changes

(API changes/additions that would affect module, install profile, and theme developers, including examples of before/after code if appropriate)
No API changes envisaged as code would be added to re-use existing functionality

Original report by [username]

// Text of original report here.
(for legacy issues whose initial post was not the issue summary)
Not required - feature request described as above.