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)

Too hacky for my liking. You are playing around with the routes etc. Just check for `params[:activity_id]` and when other things are commentable, we will check for other types of parameters like `params[:resource_id]`. Yes that's less rhobust and more hardcoded but it's better imho. Open to discuss though!