Fixed #8342 -- Removed code from the admin that assumed that you can't login...

Fixed #8342 -- Removed code from the admin that assumed that you can't login with an email address (nixed by r12634). Also refactored login code slightly to be DRY by using more of auth app's forms and views.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14769 bcc190cf-cafb-0310-a4f2-bffc1f526a37