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>

tests/mysql.ini.tmpl

@@ -124,3 +124,13 @@ INTERNAL_TOKEN = eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJuYmYiOjE0OTU1NTE2MTh9.h
 
 [packages]
 ENABLED = true
+
+[email.incoming]
+ENABLED = true
+HOST = smtpimap
+PORT = 993
+USERNAME = debug@localdomain.test
+PASSWORD = debug
+USE_TLS = true
+SKIP_TLS_VERIFY = true
+REPLY_TO_ADDRESS = incoming+%{token}@localhost