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

[CYB-272 Item submitter gets points when another user comments on item](https://jira.ets.berkeley.edu/jira/browse/CYB-272)