CiviMail: re-sending a (possibly changed) mailing

Add ability to to re-send a mailing to another group, or duplicate and change things like subject and send to another group.

Add a 'Re-send' action to the mailling browser. This defines a new mailing, but pre-filled with the selected mailing's data: the user lands directly at the 4th step of the mailing wizard (Test), with the previous steps still available to be altered by clicking the 'Previous' button.

The only tricky part here seems to be hooking the message forms (file selectors) to use the data from the database instead of files.