Allow internal ContentNode endpoint to handle baseurl parameter for browsing non-local libraries

[rtibbles]

This would be similar in motivation to the RemoteChannelViewset: https://github.com/learningequality/kolibri/blob/develop/kolibri/core/content/api.py#L1447 which is used to provide a local API endpoint for browsing the public channel API of remote Kolibris and Kolibri Studio.

This should then map the data returned from the public endpoint to data that is mapped to the current internal ContentNode configuration.

The implementation should intelligently cache queries, to avoid making more requests than necessary on the local device.