[bugfix] add Date and Message-ID headers for email (#3031)

* [bugfix] add Date and Message-ID headers for email

This should make spam filters more happy, as most of them grant some
negative score for not having those headers. Also the Date is convenient
for the user receiving the mail.

* make golangci-lint happy

internal/email/sender.go

@@ -76,11 +76,13 @@ func NewSender() (Sender, error) {
 	host := config.GetSMTPHost()
 	port := config.GetSMTPPort()
 	from := config.GetSMTPFrom()
+	msgIDHost := config.GetHost()
 
 	return &sender{
 		hostAddress: fmt.Sprintf("%s:%d", host, port),
 		from:        from,
 		auth:        smtp.PlainAuth("", username, password, host),
+		msgIDHost:   msgIDHost,
 		template:    t,
 	}, nil
 }
@@ -89,5 +91,6 @@ type sender struct {
 	hostAddress string
 	from        string
 	auth        smtp.Auth
+	msgIDHost   string
 	template    *template.Template
 }