Make the CiviMail information on sent mailings avaliable as Activity History

The information on the CiviMail mailings should be avaliable through Activity History (both the UI and APIs, and in Activity History search as well).

Insert one Activity History record per recipient per mailing - with Details link pointing to the CiviMail Mailing report for this mailing. Other activity_history properties should be populated as follows:

module = "CiviMail"
activity_type = "Send Email"
activity_summary = $subject from mailing
activity_date = $mailing date/time
activity_id = $mailing id