Initial port of External Links module to Backdrop.
This version is identical in functionality to the 1.x-1.18 release of External Links for Drupal (https://www.drupal.org/node/2309553).
This module is fully ported to Backdrop, with an upgrade path from Drupal 7. A video of porting this module was published to YouTube as an example of how to port a module. See the video at https://www.youtube.com/watch?v=by8mMhIpARA.