Remove Discussion and Advanced Modules from Content Libraries

[Kelketek]

• 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]

@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?

[Kelketek]

@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.

[e-kolpakov]

👍 with minor note.

[e-kolpakov]

One more thing - it might make sense to add python for get_component_templates. Bok choy test are required, but they are so sloooooow, so adding faster tests would benefit development cycle. If it's possible to add them not refactoring the whole thing into something completely different.

[Kelketek]

@e-kolpakov Added a python test. Getting this upstream now.

[e-kolpakov]

@Kelketek do we still need this PR?