Use FullName in Emails to address the recipient if possible (#31527)

Before we had just the plain mail address as recipient. But now we
provide additional Information for the Mail clients.

---
*Sponsored by Kithara Software GmbH*

models/user/user.go

@@ -8,6 +8,8 @@ import (
 	"context"
 	"encoding/hex"
 	"fmt"
+	"mime"
+	"net/mail"
 	"net/url"
 	"path/filepath"
 	"regexp"
@@ -413,6 +415,34 @@ func (u *User) DisplayName() string {
 	return u.Name
 }
 
+var emailToReplacer = strings.NewReplacer(
+	"\n", "",
+	"\r", "",
+	"<", "",
+	">", "",
+	",", "",
+	":", "",
+	";", "",
+)
+
+// EmailTo returns a string suitable to be put into a e-mail `To:` header.
+func (u *User) EmailTo() string {
+	sanitizedDisplayName := emailToReplacer.Replace(u.DisplayName())
+
+	// should be an edge case but nice to have
+	if sanitizedDisplayName == u.Email {
+		return u.Email
+	}
+
+	to := fmt.Sprintf("%s <%s>", sanitizedDisplayName, u.Email)
+	add, err := mail.ParseAddress(to)
+	if err != nil {
+		return u.Email
+	}
+
+	return fmt.Sprintf("%s <%s>", mime.QEncoding.Encode("utf-8", add.Name), add.Address)
+}
+
 // GetDisplayName returns full name if it's not empty and DEFAULT_SHOW_FULL_NAME is set,
 // returns username otherwise.
 func (u *User) GetDisplayName() string {