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