Remove Discussion and Advanced Components Interface From Libraries (SOL-120) (WIP)

* **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**: Studio: http://sandbox3.opencraft.com:18010/ - LMS: http://sandbox3.opencraft.com/
* **Internal code reviews PRs**: https://github.com/open-craft/edx-platform/pull/20
* **Partner information**: 3rd party-hosted open edX instance, for an edX solutions client.
This comment is misleading -- this test is actually testing that the UI to add a discussion block to a library doesn't appear. If you want to write a test to check that we cannot add a discussion block to a library, the test should make an HTTP request attempting to add the discussion block, and should verify that the HTTP response had a status code in the 4XX range and that the database hasn't changed.