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/setting/incoming_email.go

@@ -0,0 +1,73 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package setting
+
+import (
+	"fmt"
+	"net/mail"
+	"strings"
+
+	"code.gitea.io/gitea/modules/log"
+)
+
+var IncomingEmail = struct {
+	Enabled              bool
+	ReplyToAddress       string
+	TokenPlaceholder     string `ini:"-"`
+	Host                 string
+	Port                 int
+	UseTLS               bool `ini:"USE_TLS"`
+	SkipTLSVerify        bool `ini:"SKIP_TLS_VERIFY"`
+	Username             string
+	Password             string
+	Mailbox              string
+	DeleteHandledMessage bool
+	MaximumMessageSize   uint32
+}{
+	Mailbox:              "INBOX",
+	DeleteHandledMessage: true,
+	TokenPlaceholder:     "%{token}",
+	MaximumMessageSize:   10485760,
+}
+
+func newIncomingEmail() {
+	if err := Cfg.Section("email.incoming").MapTo(&IncomingEmail); err != nil {
+		log.Fatal("Unable to map [email.incoming] section on to IncomingEmail. Error: %v", err)
+	}
+
+	if !IncomingEmail.Enabled {
+		return
+	}
+
+	if err := checkReplyToAddress(IncomingEmail.ReplyToAddress); err != nil {
+		log.Fatal("Invalid incoming_mail.REPLY_TO_ADDRESS (%s): %v", IncomingEmail.ReplyToAddress, err)
+	}
+}
+
+func checkReplyToAddress(address string) error {
+	parsed, err := mail.ParseAddress(IncomingEmail.ReplyToAddress)
+	if err != nil {
+		return err
+	}
+
+	if parsed.Name != "" {
+		return fmt.Errorf("name must not be set")
+	}
+
+	c := strings.Count(IncomingEmail.ReplyToAddress, IncomingEmail.TokenPlaceholder)
+	switch c {
+	case 0:
+		return fmt.Errorf("%s must appear in the user part of the address (before the @)", IncomingEmail.TokenPlaceholder)
+	case 1:
+	default:
+		return fmt.Errorf("%s must appear only once", IncomingEmail.TokenPlaceholder)
+	}
+
+	parts := strings.Split(IncomingEmail.ReplyToAddress, "@")
+	if !strings.Contains(parts[0], IncomingEmail.TokenPlaceholder) {
+		return fmt.Errorf("%s must appear in the user part of the address (before the @)", IncomingEmail.TokenPlaceholder)
+	}
+
+	return nil
+}