CYB-246 Being liked should also score for the poster
  * adds a field on the activity model for the poster when type is an opinion
  * Activity.student_scores now sums values for poster on likes as well as for active actions
  * removing #index on likes_controller (unused)
  * requires a migration

https://jira.ets.berkeley.edu/jira/browse/CYB-246
Merge pull request #142 from coyote/score_being_liked

CYB-246 Being liked should also score for the poster