Make static e-mail template subjects scriptable. Closes #1727.

This commit introduces optional, custom `<title>` tags that can be
added to `static/email-templates/*.html` so as to make the subjectlines
fully scriptable for system e-mails and notifications.

The tag must look like this.

```
<title data-i18n>Stuff {{ .Subscriber.Name }} here!</title>
```

cmd/notifications.go

@@ -2,6 +2,8 @@ package main
 
 import (
 	"bytes"
+	"regexp"
+	"strings"
 
 	"github.com/knadh/listmonk/models"
 )
@@ -13,6 +15,10 @@ const (
 	notifSubscriberData  = "subscriber-data"
 )
 
+var (
+	reTitle = regexp.MustCompile(`(?s)<title\s*data-i18n\s*>(.+?)</title>`)
+)
+
 // notifData represents params commonly used across different notification
 // templates.
 type notifData struct {
@@ -26,18 +32,21 @@ func (app *App) sendNotification(toEmails []string, subject, tplName string, dat
 		return nil
 	}
 
-	var b bytes.Buffer
-	if err := app.notifTpls.tpls.ExecuteTemplate(&b, tplName, data); err != nil {
+	var buf bytes.Buffer
+	if err := app.notifTpls.tpls.ExecuteTemplate(&buf, tplName, data); err != nil {
 		app.log.Printf("error compiling notification template '%s': %v", tplName, err)
 		return err
 	}
+	body := buf.Bytes()
+
+	subject, body = getTplSubject(subject, body)
 
 	m := models.Message{}
 	m.ContentType = app.notifTpls.contentType
 	m.From = app.constants.FromEmail
 	m.To = toEmails
 	m.Subject = subject
-	m.Body = b.Bytes()
+	m.Body = body
 	m.Messenger = emailMsgr
 	if err := app.manager.PushMessage(m); err != nil {
 		app.log.Printf("error sending admin notification (%s): %v", subject, err)
@@ -45,3 +54,14 @@ func (app *App) sendNotification(toEmails []string, subject, tplName string, dat
 	}
 	return nil
 }
+
+// getTplSubject extrcts any custom i18n subject rendered in the given rendered
+// template body. If it's not found, the incoming subject and body are returned.
+func getTplSubject(subject string, body []byte) (string, []byte) {
+	m := reTitle.FindSubmatch(body)
+	if len(m) != 2 {
+		return subject, body
+	}
+
+	return strings.TrimSpace(string(m[1])), reTitle.ReplaceAll(body, []byte(""))
+}

cmd/public.go

@@ -590,14 +590,20 @@ func handleSelfExportSubscriberData(c echo.Context) error {
 			makeMsgTpl(app.i18n.T("public.errorTitle"), "", app.i18n.Ts("public.errorProcessingRequest")))
 	}
 
+	var (
+		subject = app.i18n.Ts("email.data.title")
+		body    = msg.Bytes()
+	)
+	subject, body = getTplSubject(subject, body)
+
 	// Send the data as a JSON attachment to the subscriber.
 	const fname = "data.json"
 	if err := app.messengers[emailMsgr].Push(models.Message{
 		ContentType: app.notifTpls.contentType,
 		From:        app.constants.FromEmail,
 		To:          []string{data.Email},
-		Subject:     app.i18n.Ts("email.data.title"),
-		Body:        msg.Bytes(),
+		Subject:     subject,
+		Body:        body,
 		Attachments: []models.Attachment{
 			{
 				Name:    fname,