Switch to storing To addresses as a slice

- Changes on-disk storage format
- Changes JSON API
- To and From values are now parsed/formatted by Go's mail.ParseAddress
  function
- Fixed bug in list-entry-template, was not escaping HTML characters
- Updated tests

smtpd/datastore.go

@@ -36,7 +36,7 @@ type Mailbox interface {
 type Message interface {
 	ID() string
 	From() string
-	To() string
+	To() []string
 	Date() time.Time
 	Subject() string
 	RawReader() (reader io.ReadCloser, err error)