CYB-305 Teacher earns points for creating discussion assignments

* Teacher shouldn't earn points.  This is implemented by using an additional scoring flag (assigned_discussion) to Activity.  Rather than just not score those discussions, track if the replies have been scored.

https://jira.ets.berkeley.edu/jira/browse/CYB-305
Merge pull request #196 from coyote/exclude_assigned_discussions_from_scoring

CYB-305 Teacher earns points for creating discussion assignments