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 {

models/user/user_test.go

@@ -529,6 +529,29 @@ func Test_NormalizeUserFromEmail(t *testing.T) {
 	}
 }
 
+func TestEmailTo(t *testing.T) {
+	testCases := []struct {
+		fullName string
+		mail     string
+		result   string
+	}{
+		{"Awareness Hub", "awareness@hub.net", "Awareness Hub <awareness@hub.net>"},
+		{"name@example.com", "name@example.com", "name@example.com"},
+		{"Hi Its <Mee>", "ee@mail.box", "Hi Its Mee <ee@mail.box>"},
+		{"Sinéad.O'Connor", "sinead.oconnor@gmail.com", "=?utf-8?q?Sin=C3=A9ad.O'Connor?= <sinead.oconnor@gmail.com>"},
+		{"Æsir", "aesir@gmx.de", "=?utf-8?q?=C3=86sir?= <aesir@gmx.de>"},
+		{"new😀user", "new.user@alo.com", "=?utf-8?q?new=F0=9F=98=80user?= <new.user@alo.com>"},
+		{`"quoted"`, "quoted@test.com", "quoted <quoted@test.com>"},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(testCase.result, func(t *testing.T) {
+			testUser := &user_model.User{FullName: testCase.fullName, Email: testCase.mail}
+			assert.EqualValues(t, testCase.result, testUser.EmailTo())
+		})
+	}
+}
+
 func TestDisabledUserFeatures(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())