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.
* Existing code elsewhere should be safe from the default (which is false, so those items count).  Also existing records will have the field set to the default, 'false'.

[CYB 305 Teacher earns points for creating discussion assignments](https://jira.ets.berkeley.edu/jira/browse/CYB-305)