Basic discussions forum framework

Adds the basic structure for the Discussions MFE around which future development
will happen.

src/discussions/posts/data/selectors.js

@@ -0,0 +1,4 @@
+/* eslint-disable import/prefer-default-export */
+export const selectCourseThreads = topicId => state => state.threads.threads[topicId] || [];
+
+export const courseTopicsStatus = state => state.topics.status;