Add ability to move nested components into `addons/` for grouping.

[rwjblue]

Given:

app/
  components/
     modals/
       leave-editor.js
       delete-post.js
       transfer-owner.js

Migrate to:

addons/
  modals/
    leave-editor/
      component.js
    delete-post/
      component.js
    transfer-owner/
      component.js

This will require template rewriting from {{modals/leave-editor to {{modals::leave-editor.