implement imap savedate extension, rfc 8514

it makes a new field available on stored messages. not when they they were
received (over smtp) or appended to the mailbox (over imap), but when they were
last "saved" in the mailbox. copy/move of a message (eg to the trash) resets
the "savedate" value. this helps implement "remove messages from trash after X
days".

webops/xops.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"os"
 	"sort"
+	"time"
 
 	"golang.org/x/exp/maps"
 
@@ -381,6 +382,7 @@ func (x XOps) MessageMoveTx(ctx context.Context, log mlog.Log, acc *store.Accoun
 	var mbSrc store.Mailbox
 
 	keywords := map[string]struct{}{}
+	now := time.Now()
 
 	for _, mid := range messageIDs {
 		m := x.messageID(ctx, tx, mid)
@@ -436,6 +438,7 @@ func (x XOps) MessageMoveTx(ctx context.Context, log mlog.Log, acc *store.Accoun
 		m.ModSeq = modseq
 		mbDst.UIDNext++
 		m.JunkFlagsForMailbox(mbDst, conf)
+		m.SaveDate = &now
 		err = tx.Update(&m)
 		x.Checkf(ctx, err, "updating moved message in database")