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>

custom/conf/app.example.ini

@@ -1664,6 +1664,47 @@ ROUTER = console
 ;; convert \r\n to \n for Sendmail
 ;SENDMAIL_CONVERT_CRLF = true
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;[email.incoming]
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;
+;; Enable handling of incoming emails.
+;ENABLED = false
+;;
+;; The email address including the %{token} placeholder that will be replaced per user/action.
+;; Example: incoming+%{token}@example.com
+;; The placeholder must appear in the user part of the address (before the @).
+;REPLY_TO_ADDRESS =
+;;
+;; IMAP server host
+;HOST =
+;;
+;; IMAP server port
+;PORT =
+;;
+;; Username of the receiving account
+;USERNAME =
+;;
+;; Password of the receiving account
+;PASSWORD =
+;;
+;; Whether the IMAP server uses TLS.
+;USE_TLS = false
+;;
+;; If set to true, completely ignores server certificate validation errors. This option is unsafe.
+;SKIP_TLS_VERIFY = true
+;;
+;; The mailbox name where incoming mail will end up.
+;MAILBOX = INBOX
+;;
+;; Whether handled messages should be deleted from the mailbox.
+;DELETE_HANDLED_MESSAGE = true
+;;
+;; Maximum size of a message to handle. Bigger messages are ignored. Set to 0 to allow every size.
+;MAXIMUM_MESSAGE_SIZE = 10485760
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;[cache]