Feature/81 comments

Completed comment feature using polymorphic attribute.
     -comment actions: create, index.
     -users can comment on different type of activities on: (activities/show path)

There's already a user_id when the comments table is created ( https://github.com/lighthouse-labs/laser_shark/pull/43/files#diff-99e9750697ee809a7da2cc76bb4b222bR6 ) - I imagine this would fail (haven't tried it out yet)