Refactor the News media gallery's view controller

Rename the News module's gallery image view controller and separate out its XIB from the storyboard
More tweaks to the update mechanism

Keep all the state information local to the update block and add a completion blocked which is fired at the appropriate time (instead of keeping the state in an ivar)
Clean up the CoreData accesses

Ensure all the operations take place on the proper queue (and in the proper MOC)
Hook up the modal media gallery

Conditionally handle the segue to the media gallery and setup images for the gallery to page through
Configure the gallery's page view controller

Since the image views are loaded on-demand from URLs, we should not need to implement the UIPageViewControllerDataSource protocol.