Remove discussions backend code

[rhysyngsun]

We want to remove the backend code related to the legacy discussions features.

• This should not remove the newer channel fields functionality.
• This should remove any code dependencies on Reddit.
• The cassette/ and factory_data/ directories contain test data related to Reddit that should be removed.
• (Added by Chris C) Some URL routes in open_discussions.urls were tied to reddit-related tests and should be removed.