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.
Not in import_channel, but the base method still looks for it. There is a lot of clean up to do in terms of extracting out khanspecific filtering from the contentload command.