Add List-Unsubscribe header (#17804)

Fixes #13283

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/mailer/mail.go

@@ -348,7 +348,7 @@ func generateAdditionalHeaders(ctx *mailCommentContext, reason string, recipient
 		// https://datatracker.ietf.org/doc/html/rfc2369
 		"List-Archive": fmt.Sprintf("<%s>", repo.HTMLURL()),
 		//"List-Post": https://github.com/go-gitea/gitea/pull/13585
-		//"List-Unsubscribe": https://github.com/go-gitea/gitea/issues/10808, https://github.com/go-gitea/gitea/issues/13283
+		"List-Unsubscribe": ctx.Issue.HTMLURL(),
 
 		"X-Gitea-Reason":            reason,
 		"X-Gitea-Sender":            ctx.Doer.DisplayName(),

services/mailer/mail_test.go

@@ -218,6 +218,7 @@ func TestGenerateAdditionalHeaders(t *testing.T) {
 	expected := map[string]string{
 		"List-ID":                   "user2/repo1 <repo1.user2.localhost>",
 		"List-Archive":              "<https://try.gitea.io/user2/repo1>",
+		"List-Unsubscribe":          "https://try.gitea.io/user2/repo1/issues/1",
 		"X-Gitea-Reason":            "dummy-reason",
 		"X-Gitea-Sender":            "< U<se>r Tw<o > ><",
 		"X-Gitea-Recipient":         "Test",