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
CYB-323 User likes/dislikes only count *once* per gallery item

* By tracking all activity to likes/dislikes, all actions were being counted in the return counts for gallery/index action,
   and so also on the displayed gallery page.  Instead, only count each person's action once.

https://jira.ets.berkeley.edu/jira/browse/CYB-323
Merge pull request #197 from coyote/single_count_likes_per_gallery_item

CYB-323 User likes/dislikes only count *once* per gallery item