Removed the ability to add Discussion and advanced components to Content Libraries.
Merge pull request #6177 from open-craft/content_libraries/12-remove-discussion

Remove Discussion and Advanced Components Interface From Libraries (SOL-120) (WIP)
Three levels of user permissions for content libraries:
Admin ("Instructor") - Can edit and assign permissions to other users
Normal ("Staff") - Can edit
User - Can view the library and use content from it but cannot edit it or its blocks.
Unit tests for content library permissions
Update the REST API course_team_handler to support library roles, fix bugs
Basic UI for managing library users with limited editing functionality (shared with course code)
Acceptance & Jasmine tests for library permissions editor