Added a password reset link to default admin login page if a password reset URL is available.

You no longer have to override the admin login page just to get that link to
appear!

Also provided much better docs for how to plug in the provided password
reset mechanism with minimum effort.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17266 bcc190cf-cafb-0310-a4f2-bffc1f526a37