Discussion runtime templates

This PR includes:

- The addition of LmsCourse and LmsUser mixins to expose the course and loggued user object to the xblock.
- The addition of DiscussionService to expose the required lms context for the template rendering.
- The removal of many templates in the LMS. They are now read from the discussion_app. 

The official PR is at: https://github.com/edx/edx-platform/pull/3829