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>

modules/util/pack.go

@@ -0,0 +1,33 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"bytes"
+	"encoding/gob"
+)
+
+// PackData uses gob to encode the given data in sequence
+func PackData(data ...interface{}) ([]byte, error) {
+	var buf bytes.Buffer
+	enc := gob.NewEncoder(&buf)
+	for _, datum := range data {
+		if err := enc.Encode(datum); err != nil {
+			return nil, err
+		}
+	}
+	return buf.Bytes(), nil
+}
+
+// UnpackData uses gob to decode the given data in sequence
+func UnpackData(buf []byte, data ...interface{}) error {
+	r := bytes.NewReader(buf)
+	enc := gob.NewDecoder(r)
+	for _, datum := range data {
+		if err := enc.Decode(datum); err != nil {
+			return err
+		}
+	}
+	return nil
+}