in account web page, allow user to configure rulesets for delivery

for example, by matching incoming messags on smtp mail from, verified domains
(spf/dkim), headers. then delivering to a configured mailbox. for mailing
lists, if a verified domain matches, regular spam checks can be skipped.

this was already possible by editing the configuration file, but only admins
can edit that file. now users can manage their own rulesets.

mox-/admin.go

@@ -703,6 +703,51 @@ func AddressRemove(ctx context.Context, address string) (rerr error) {
 	return nil
 }
 
+// DestinationSave updates a destination for an account and reloads the configuration.
+func DestinationSave(ctx context.Context, account, destName string, newDest config.Destination) (rerr error) {
+	log := xlog.WithContext(ctx)
+	defer func() {
+		if rerr != nil {
+			log.Errorx("saving destination", rerr, mlog.Field("account", account), mlog.Field("destname", destName), mlog.Field("destination", newDest))
+		}
+	}()
+
+	Conf.dynamicMutex.Lock()
+	defer Conf.dynamicMutex.Unlock()
+
+	c := Conf.Dynamic
+	acc, ok := c.Accounts[account]
+	if !ok {
+		return fmt.Errorf("account not present")
+	}
+
+	if _, ok := acc.Destinations[destName]; !ok {
+		return fmt.Errorf("destination not present")
+	}
+
+	// Compose new config without modifying existing data structures. If we fail, we
+	// leave no trace.
+	nc := c
+	nc.Accounts = map[string]config.Account{}
+	for name, a := range c.Accounts {
+		nc.Accounts[name] = a
+	}
+	nd := map[string]config.Destination{}
+	for dn, d := range acc.Destinations {
+		nd[dn] = d
+	}
+	nd[destName] = newDest
+	nacc := nc.Accounts[account]
+	nacc.Destinations = nd
+	nc.Accounts[account] = nacc
+
+	if err := writeDynamic(ctx, nc); err != nil {
+		return fmt.Errorf("writing domains.conf: %v", err)
+	}
+	log.Info("destination saved", mlog.Field("account", account), mlog.Field("destname", destName))
+	return nil
+}
+
 // ClientConfig holds the client configuration for IMAP/Submission for a
 // domain.
 type ClientConfig struct {