[pkg/mailer] Revert to "To" and add Subject prefix

- Using Bcc instead of To increases the spaminess of our emails
(according to mail-tester.com).
- SubjectPrefix from upstream

pkg/mailer/mailer.go

@@ -33,8 +33,8 @@ func NewMessageFrom(to []string, from, subject, htmlBody string) *Message {
 
 	msg := gomail.NewMessage()
 	msg.SetHeader("From", from)
-	msg.SetHeader("Bcc", to...)
-	msg.SetHeader("Subject", subject)
+	msg.SetHeader("To", to...)
+	msg.SetHeader("Subject", setting.MailService.SubjectPrefix+subject)
 	msg.SetDateHeader("Date", time.Now())
 
 	contentType := "text/html"