Add support for incoming emails (#22056)

closes #13585
fixes #9067
fixes #2386
ref #6226
ref #6219
fixes #745

This PR adds support to process incoming emails to perform actions.
Currently I added handling of replies and unsubscribing from
issues/pulls. In contrast to #13585 the IMAP IDLE command is used
instead of polling which results (in my opinion 😉) in cleaner code.

Procedure:
- When sending an issue/pull reply email, a token is generated which is
present in the Reply-To and References header.
- IMAP IDLE waits until a new email arrives
- The token tells which action should be performed

A possible signature and/or reply gets stripped from the content.

I added a new service to the drone pipeline to test the receiving of
incoming mails. If we keep this in, we may test our outgoing emails too
in future.

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

services/mailer/mailer.go

@@ -36,6 +36,7 @@ type Message struct {
 	FromAddress     string
 	FromDisplayName string
 	To              []string
+	ReplyTo         string
 	Subject         string
 	Date            time.Time
 	Body            string
@@ -47,6 +48,9 @@ func (m *Message) ToMessage() *gomail.Message {
 	msg := gomail.NewMessage()
 	msg.SetAddressHeader("From", m.FromAddress, m.FromDisplayName)
 	msg.SetHeader("To", m.To...)
+	if m.ReplyTo != "" {
+		msg.SetHeader("Reply-To", m.ReplyTo)
+	}
 	for header := range m.Headers {
 		msg.SetHeader(header, m.Headers[header]...)
 	}