Comment index view is (re)done, taking into account advice from Antoine de Saint-Exupery.

- `app/views/comments/index.html.haml` is now positively Spartan; the loop which became an ugly content div wrapper around two decorator calls has become instead a single decorator method call.
- The ugliness formerly in the view has been pushed into a new decorator method, `CommentDecorator#format_entire_comment`.
- Since the previously-public `#format_content` and `#format_header` methods on `CommentDecorator` are no longer public, their specs have been removed from `comment_decorator_spec.rb`