Work on a stable Radix 3.x release and prepare for Panopoly integration

Radix 3.x is looking good and now being used on a lot of sites (https://drupal.org/project/usage/radix). Let's start planning a stable 3.0 release.

This issue can be used as the parent issue for Panopoly integration issues.

Hitlist

#2220967: Make sure Radix works well with Panels everywhere and write the documentation for this.
#2220973: Add a _term.scss partial for terms pages
#2223365: Use input group component for Input Format Widget
#2223703: Drush Radix should work without enabling Radix theme
#2224149: CDN assets should be optional
#2505249: Add 'Help' region to Radix