Allow admin to save expired broadcast messages

For auditing and documentation purposes, it would be helpful if admins could save expired broadcast messages, including a record of when they were enabled and when they were deleted.  This functionality would be useful in cases where one or more users have questions about information that was previously broadcast.  For example, a broadcast may be sent out regarding planned maintenance, and if some users just click through the message, they may then have a question about whether they were notified.