Add setting to force plain SMTP authentication

[jdemoor]

A wrongly configured SMTP server that requires authentication can advertise CRAM-MD5 as an accepted authentication method but fail when the client tries to use it. Most email clients would try other methods after the failure, but Python's smtplib just raises SMTPAuthException. This makes sending emails from Django with such servers impossible.

The attached patch adds the EMAIL_FORCE_PLAIN_AUTH setting which, if True, forces the use of the PLAIN method in django.core.mail.SMTPConnection.open().

[Jacob]

I don't like adding more code to Django just to work around other broken behavior; we can't be responsible for all the problems in other people's code. In this case, we're talking *both* a misconfigured SMTP server *and* a problem with smtplib.