Fixed #9958: split the `CommentForm` into a set of smaller forms. This for better encapsulation, but also so that it's easier for subclasses to get at the pieces they might need. Thanks to Thejaswi Puthraya.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10110 bcc190cf-cafb-0310-a4f2-bffc1f526a37