Update export channel and export content functionality to create a single queryable database that can be used to preview the exported library

[rtibbles]

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Currently export channel and export content just export the raw channel database and content to the drive. In addition, it should be creating a centralized record store (essentially a 'content only' Kolibri database) that has all of the exported channels, and all appropriate annotations for all of the exported resources.

This will require some careful design of migrations and the dynamic database router that will read these databases in order to prevent errors occurring when schema changes happen over different versions of Kolibri.