If the user has abandoned the link or dwelled on a different link, then
don't make the API request.
Changes:
* Fix a bug in the linkDwell reducer where the activeLink was always
being set to undefined.
* Add the private fetch action creator.
* Make the linkDwell action dispatch the result of the fetch action
creator after 500ms.
Supporting changes:
* Make the link* action creators take DOM elements rather than jQuery
instances so that:
1. Testing whether the state tree has changed is an identity
comparison.
2. jQuery instances are constructed only when required.
* Document the ext.popups.Gateway type.
* Document the Redux.Store type.
* Rename the "previews-page-title" data attribute to
"page-previews-title" to avoid confusion.
Change-Id: I0b1cf3337a6f8d6450ad2bd127cb292ebb73af4f
