Expose the caption_file information in the PublicationPresenter

Captions have been added to `VideoEdition` in the form of the
`caption_file` key. Expose this key on the PublicationPresenter so
that we can use the linked XML file in our video html view.
Be specific about where we expect to see the XML caption in the DOM
Merge pull request #329 from alphagov/caption-files-for-video-views

Caption files for video views