CYB-59 Add a CSV export for points

* now uses two Activity records; one for the liker, and one for the poster of the item liked
* 2 new action types:  GetALike, GetADislike
* 'MarkNeutral' is no longer a pseudo-status (it is gone)

https://jira.ets.berkeley.edu/jira/browse/CYB-59
Merge pull request #179 from coyote/csv_export

CYB-59 Add a CSV export for points
CYB-272 Item submitter gets points when another user comments on item

 * Activity entries created for original poster and for commenter (if they are not the same)
 * Uses new activity type ("GetAComment") which has an English string in config/locales/en.yml
Merge pull request #181 from coyote/commenter_scoring

CYB-272 Item submitter gets points when another user comments on item
CYB-215 Engagement index: opt-out students are still visible to everybody

* added several scopes to Activity, and #visible_to filters on Student and Activity
* increased spec coverage on Activity and EngagementIndexController
* some refactoring of the EngagementIndexController to reduce method size (still too big)

https://jira.ets.berkeley.edu/jira/browse/CYB-215