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.
Function that iterates over the different content (Video, Exercise, Content) caches, and replaces a list of 'related_content' filenames with the generated unique ids.