Enhance non-local ContentNode viewset to allow browsing of up to date drive based libraries

[rtibbles]

In addition to the baseurl GET param (e.g. https://github.com/learningequality/kolibri/blob/develop/kolibri/core/content/api.py#L1571), we should also add a drive_id to allow for querying and previewing metadata contained on a local drive.

This would then require using a dynamic database router in Django in order to run the appropriate queries against a database that has been exported by a newer version of Kolibri.