Fix for lack of date header in new list notification emails

This is a fix for Bug 1212066, where the notification
email sent to the Mailman site owner after a new list
is created using the Web UI does not include a Date
header as required by RFC 2822. The underlying issue
appears to be that the notification email is being sent
with an outdated or incorrect method (outq.enqueue
instead of msg.send). I simply took the same code that
is used in the newlist script to do this and added it
here, along with the code that it ensures that the
header uses the site owner list's preferred language.

This patch also includes a fix for a bug that has not
been reported, in which notification emails sent upon
list removal experience the same issue.