CiviMail: Archived mailings

From U.S. PIRG comments:

Option to archive old mailings in a separate area of CiviMail so they don't clutter up the interface, but still retain the ability to see / search them in that "archive" section.

• Add an is_archived column to civicrm_mailing and list only unarchived mailings by default in listings
• Also add an Archived Mailings in the menu which would do the opposite
• All mailings should be accessed by their direct URLs as they are now (so no change here; these screens should not check for the is_archived status).