(WIP) Content Libraries - Library Course XBlock

@antoviaque @explorerleslie @cahrens

**Background:** This PR contains LibraryContent XModule that allows displaying library content in a course. It is a part of SOL-5, SOL-6, SOL-7.

**Discussion:** 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.

**Partner information:** 3rd party-hosted open edX instance, for an edX solutions client.

**Merge deadline:** Dec 12

**Dependencies:** Requires #6046, which is already merged.