6800299 comments ability to reply

    Replying to comments in content planner
    https://www.pivotaltracker.com/story/show/68002996

    - added base reply comment functionality
    - added reply email notification
    - added permission policy for reply
      - added comment replies page
        - this page displays parent comment and all replies
        - user can add one more reply on this page
        - we will redirect to this page from mailer
      - added Rspec checks for:
        - review replies
        - reply from commentable association
        - reply from comment replies page
        - covered reply comment email notification with Rspec