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.
@e-kolpakov Don't know, the issue on Jira just says to do it, so I did. Disabling `discussion-forum` shouldn't be needed, since it's an advanced component and advanced components won't be used.