Include username in email headers

Emails from Gitea comments do not contain the username of the commenter
anywhere, only their display name, so it is not possible to verify who
made a comment from the email itself:

	From: "Alice" <email@gitea>
	X-Gitea-Sender: Alice
	X-Gitea-Recipient: Bob
	X-GitHub-Sender: Alice
	X-GitHub-Recipient: Bob

	This comment looks like it's from @alice.

The X-Gitea/X-GitHub headers also use display names, which is not very
reliable for filtering, and inconsistent with GitHub's behavior:

	X-GitHub-Sender: lunny
	X-GitHub-Recipient: gwymor

This change includes both the display name and username in the From
header, and switches the other headers from display name to username:

	From: "Alice (@fakeAlice)" <email@gitea>
	X-Gitea-Sender: fakealice
	X-Gitea-Recipient: bob
	X-GitHub-Sender: fakealice
	X-GitHub-Recipient: bob

	This comment looks like it's from @alice.

Author: gwymor

services/mailer/mail.go

@@ -310,7 +310,13 @@ func composeIssueCommentMessages(ctx *mailCommentContext, lang string, recipient
 
 	msgs := make([]*Message, 0, len(recipients))
 	for _, recipient := range recipients {
-		msg := NewMessageFrom(recipient.Email, ctx.Doer.DisplayName(), setting.MailService.FromEmail, subject, mailBody.String())
+		msg := NewMessageFrom(
+			recipient.Email,
+			fmt.Sprintf("%s (@%s)", ctx.Doer.DisplayName(), ctx.Doer.Name),
+			setting.MailService.FromEmail,
+			subject,
+			mailBody.String(),
+		)
 		msg.Info = fmt.Sprintf("Subject: %s, %s", subject, info)
 
 		msg.SetHeader("Message-ID", msgID)
@@ -394,8 +400,8 @@ func generateAdditionalHeaders(ctx *mailCommentContext, reason string, recipient
 
 		"X-Mailer":                  "Gitea",
 		"X-Gitea-Reason":            reason,
-		"X-Gitea-Sender":            ctx.Doer.DisplayName(),
-		"X-Gitea-Recipient":         recipient.DisplayName(),
+		"X-Gitea-Sender":            ctx.Doer.Name,
+		"X-Gitea-Recipient":         recipient.Name,
 		"X-Gitea-Recipient-Address": recipient.Email,
 		"X-Gitea-Repository":        repo.Name,
 		"X-Gitea-Repository-Path":   repo.FullName(),
@@ -404,8 +410,8 @@ func generateAdditionalHeaders(ctx *mailCommentContext, reason string, recipient
 		"X-Gitea-Issue-Link":        ctx.Issue.HTMLURL(),
 
 		"X-GitHub-Reason":            reason,
-		"X-GitHub-Sender":            ctx.Doer.DisplayName(),
-		"X-GitHub-Recipient":         recipient.DisplayName(),
+		"X-GitHub-Sender":            ctx.Doer.Name,
+		"X-GitHub-Recipient":         recipient.Name,
 		"X-GitHub-Recipient-Address": recipient.Email,
 
 		"X-GitLab-NotificationReason": reason,

services/mailer/mail_test.go

@@ -239,16 +239,16 @@ func TestGenerateAdditionalHeaders(t *testing.T) {
 	doer, _, issue, _ := prepareMailerTest(t)
 
 	ctx := &mailCommentContext{Context: context.TODO() /* TODO: use a correct context */, Issue: issue, Doer: doer}
-	recipient := &user_model.User{Name: "Test", Email: "test@gitea.com"}
+	recipient := &user_model.User{Name: "test", Email: "test@gitea.com"}
 
 	headers := generateAdditionalHeaders(ctx, "dummy-reason", recipient)
 
 	expected := map[string]string{
 		"List-ID":                   "user2/repo1 <repo1.user2.localhost>",
 		"List-Archive":              "<https://try.gitea.io/user2/repo1>",
 		"X-Gitea-Reason":            "dummy-reason",
-		"X-Gitea-Sender":            "< U<se>r Tw<o > ><",
-		"X-Gitea-Recipient":         "Test",
+		"X-Gitea-Sender":            "user2",
+		"X-Gitea-Recipient":         "test",
 		"X-Gitea-Recipient-Address": "test@gitea.com",
 		"X-Gitea-Repository":        "repo1",
 		"X-Gitea-Repository-Path":   "user2/repo1",