Rewrite delivery of issue and comment mails (#9009)

* Mail issue subscribers, rework the function

* Simplify a little more

* Fix unused variable

* Refactor mail delivery to avoid heavy load on server

* Avoid splitting into too many goroutines

* Fix comments and optimize GetMaileableUsersByIDs()

* Fix return on errors

services/mailer/mailer.go

@@ -295,9 +295,18 @@ func NewContext() {
 	go processMailQueue()
 }
 
-// SendAsync send mail asynchronous
+// SendAsync send mail asynchronously
 func SendAsync(msg *Message) {
 	go func() {
 		mailQueue <- msg
 	}()
 }
+
+// SendAsyncs send mails asynchronously
+func SendAsyncs(msgs []*Message) {
+	go func() {
+		for _, msg := range msgs {
+			mailQueue <- msg
+		}
+	}()
+}