Add a system to bulk import from a manifest file

[dylanmccall]

In kolibri-explore-plugin, add a system to discover content manifest files placed in a directory named content/bulk-import, and import them when Kolibri is initializing. This is specified as part of https://github.com/endlessm/endless-key-content-private/issues/102. This system will make it easier for us to produce deployments of Endless Key with pre-loaded content.

[dsd]

I support your 2 day exploration here but please keep it limited to that as we need to shift to new projects better aligned with our new strategy. (I am assuming that #899 will be an adequate solution for now)

Would the approach you are exploring here also require significant work on the image builder front?