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 as well as for active actions
  * removing #index on likes_controller (unused)
  * requires a migration

[CYB 246 Being liked should also score for the poster](https://jira.ets.berkeley.edu/jira/browse/CYB-246)