Fix new-user notification on django < 1.3

Review Request #2404 — Created June 19, 2011 and submitted — Latest diff uploaded

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Fix new-user notification on django < 1.3

The new-user notification email sent both text and html versions of the email,
but the mail_admins function didn't get an html parameter until 1.3. Since we
don't yet require this, we shouldn't rely on it.
Ran unit tests.