The comments app was unconditionally accessing various settings that didn't

exist in Django's global settings. Changed those accesses to conditional
lookups with default fallbacks.

The comment_test tests now pass without needing to add any extra settings.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8573 bcc190cf-cafb-0310-a4f2-bffc1f526a37