[MM-36872] add threading headers to notification emails (#18232)

* [MM-36872] Adding headers for mail threading

* MM-36872 Add message id and mail threading headers

* MM-36872 Add message id and mail threading headers

* MM-36872 Add msgId, inreplyto, references headers

* Fixing comment issues

* Applying gofmt lint fixes

* Applying gofmt lint fixes

* MM-36872: Updates required from merge.

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Martin Kraft <martin@upspin.org>

app/admin.go

@@ -231,7 +231,7 @@ func (a *App) TestEmail(userID string, cfg *model.Config) *model.AppError {
 	T := i18n.GetUserTranslations(user.Locale)
 	license := a.Srv().License()
 	mailConfig := a.Srv().MailServiceConfig()
-	if err := mail.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, ""); err != nil {
+	if err := mail.SendMailUsingConfig(user.Email, T("api.admin.test_email.subject"), T("api.admin.test_email.body"), mailConfig, license != nil && *license.Features.Compliance, "", "", "", ""); err != nil {
 		return model.NewAppError("testEmail", "app.admin.test_email.failure", map[string]interface{}{"Error": err.Error()}, "", http.StatusInternalServerError)
 	}