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_test.go

@@ -0,0 +1,28 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package util
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestPackAndUnpackData(t *testing.T) {
+	s := "string"
+	i := int64(4)
+	f := float32(4.1)
+
+	var s2 string
+	var i2 int64
+	var f2 float32
+
+	data, err := PackData(s, i, f)
+	assert.NoError(t, err)
+
+	assert.NoError(t, UnpackData(data, &s2, &i2, &f2))
+	assert.NoError(t, UnpackData(data, &s2))
+	assert.Error(t, UnpackData(data, &i2))
+	assert.Error(t, UnpackData(data, &s2, &f2))
+}