when moving a message out of a Rejects mailbox, mark the message as "not seen" so stands out in the destination mailbox (e.g. inbox)

we set the flag both for move in imap and in webmail.

this also ensures the "MailboxDestinedID", used for per-mailbox reputation
analysis, is set in more reject-situations. before this change, some rejects
(such as based on DMARC reject) wouldn't result in reputation being used after
having been moved the message out of the rejects mailbox.

in the future, we need more tests for scenario's like this...

for issue #63 reported by x8x
may also help with issue #64

webmail/api.go

@@ -844,6 +844,7 @@ func (Webmail) MessageMove(ctx context.Context, messageIDs []int64, mailboxID in
 					// is used for reputation calculation during future deliveries.
 					m.MailboxOrigID = m.MailboxDestinedID
 					m.IsReject = false
+					m.Seen = false
 				}
 				m.UID = mbDst.UIDNext
 				m.ModSeq = modseq