Refactor suppression of deleted messages into MessagingController.

Re-enable instant deletes in MessageView

Deleted message suppression should be generalized into a flag cache,
serving Seen, Flagged, and Answered flags, as well.

Copy message to Trash still needs work, so that no duplicates are ever
visible and server-sync is more clean.  This will be especially
important, as the same code will be needed for the future "message
copy/move" facility.