Remove Discussion and Advanced Modules from Content Libraries

* **Background**: This PR removes the ability to add Advanced or Discussion modules to a content library, restricting the number of supported components to start with.
* **Jira tickets**: Implements SOL-120
* **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/6046 (merged, in feature branch https://github.com/edx/edx-platform/commits/content-libraries)
* **Sandbox URL**: TBD
* **Internal code reviews PRs**: (This)
* **Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.
@Kelketek Might want adding `discussion-forum` to prevent adding discussion XBlock. Not sure if it's really needed though. What was the reason behind disabling discussion XModule?