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.
Currently just adds an absolute path, so these links are handled by normal navigation (which gives a page reload) - this could be refactored to work in the same way as the links in the topic sidebar.