Content import additions

Summary of changes:
* Inspects media files for metadata during content import
* Inspects json metadata for related content files
* Adds starting template for related content
* Updates 'learn' router scheme to be namespaced by channel

@jamalex @aronasorman please review.
We could make the channel part of the url argument, so we can call this as `self.reverse("learn", channel=settings.CHANNEL)` instead of concatenating strings.