Allow admin to modify message template used for Membership Renewal Reminders

I don't think there's any reason to prevent admin from selecting a different message template for renewal reminders for a given membership type.

Also fixed sort on Message template select fields - should sort by title not by id.

http://forum.civicrm.org/index.php/topic,7345.msg50267.html#msg50267

NOTE: Allowing admins to change the Renewal Reminder Days field is NOT part of this fix - since it would require a mass update to the existing membership records - reminder_date field.