Fixed #15603 -- Changed the traceback error e-mails not to use HTML by...

Fixed #15603 -- Changed the traceback error e-mails not to use HTML by default. It's now configurable with an 'include_html' parameter to AdminEmailHandler. Thanks, kmtracey

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15850 bcc190cf-cafb-0310-a4f2-bffc1f526a37