Platform hosted discussion XBlock

This is a PR for discussion XBlock hosted in edx-platform repository.

The idea of hosting discussion XBlock in edx-platform belongs to @jimabramson and @andy-armstrong 

* Affected components: LMS (opt-in), Studio (opt-in)
* Affected users: unknown, most likely none
* Partner information: 3rd party-hosted open edX instance, for an edX solutions client (so will need to get some sort of solution merged in)
* Merge deadline: Nov 3 (soft requirement to minimize code drift, we maintain it on the client fork in the meantime)

Related:

* https://github.com/edx/edx-platform/pull/3829
* https://github.com/edx-solutions/xblock-discussion/pull/28
Internationalize "Discussion Target"