[PORT] Use FullName in Emails to address the recipient if possible (gitea#31527) (#4516)

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

---
Porting information:

- Two behavior changes are noted with this patch, the display name is now always quoted although in some scenarios unnecessary it's a safety precaution of Go. B encoding is used when certain characters are present as they aren't 'legal' to be used as a display name and Q encoding would still show them and B encoding needs to be used, this is now done by Go's `address.String()`.
- Update and add new unit tests.

Co-authored-by: 6543 <6543@obermui.de>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/4516
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/user/user.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"encoding/hex"
 	"fmt"
+	"net/mail"
 	"net/url"
 	"path/filepath"
 	"regexp"
@@ -439,6 +440,33 @@ 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
+	}
+
+	address, err := mail.ParseAddress(fmt.Sprintf("%s <%s>", sanitizedDisplayName, u.Email))
+	if err != nil {
+		return u.Email
+	}
+
+	return address.String()
+}
+
 // GetDisplayName returns full name if it's not empty and DEFAULT_SHOW_FULL_NAME is set,
 // returns username otherwise.
 func (u *User) GetDisplayName() string {