Add support for Import and Export of Libraries

* **Background**: This PR enables a user to export their Content libraries to a gzipped tarball, much like they can export a course, and import them similarly.
* **Jira tickets**: Implements SOL-82
* **Discussions**: Architecture discussed extensively on the wiki and in meetings, then the revised proposal was presented to the Arch Council on Oct. 21 and given thumbs up.
* **Dependencies**: https://github.com/edx/edx-platform/pull/6155
* **Sandbox URL**: TBD
* **Internal code reviews PRs**: (This)
* **Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.