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>

go.mod

@@ -20,11 +20,13 @@ require (
 	github.com/buildkite/terminal-to-html/v3 v3.7.0
 	github.com/caddyserver/certmagic v0.17.2
 	github.com/chi-middleware/proxy v1.1.1
-	github.com/denisenkom/go-mssqldb v0.12.3
+	github.com/denisenkom/go-mssqldb v0.12.2
+	github.com/dimiro1/reply v0.0.0-20200315094148-d0136a4c9e21
 	github.com/djherbis/buffer v1.2.0
 	github.com/djherbis/nio/v3 v3.0.1
 	github.com/dustin/go-humanize v1.0.0
 	github.com/editorconfig/editorconfig-core-go/v2 v2.5.1
+	github.com/emersion/go-imap v1.2.1
 	github.com/emirpasic/gods v1.18.1
 	github.com/ethantkoenig/rupture v1.0.1
 	github.com/felixge/fgprof v0.9.3
@@ -58,6 +60,7 @@ require (
 	github.com/hashicorp/golang-lru v0.6.0
 	github.com/huandu/xstrings v1.4.0
 	github.com/jaytaylor/html2text v0.0.0-20211105163654-bc68cce691ba
+	github.com/jhillyerd/enmime v0.10.1
 	github.com/json-iterator/go v1.1.12
 	github.com/kballard/go-shellquote v0.0.0-20180428030007-95032a82bc51
 	github.com/keybase/go-crypto v0.0.0-20200123153347-de78d2cb44f4
@@ -145,6 +148,7 @@ require (
 	github.com/blevesearch/zapx/v15 v15.3.8 // indirect
 	github.com/boombuler/barcode v1.0.1 // indirect
 	github.com/bradfitz/gomemcache v0.0.0-20190913173617-a41fca850d0b // indirect
+	github.com/cention-sany/utf7 v0.0.0-20170124080048-26cad61bd60a // indirect
 	github.com/cespare/xxhash/v2 v2.1.2 // indirect
 	github.com/cloudflare/circl v1.2.0 // indirect
 	github.com/couchbase/go-couchbase v0.0.0-20210224140812-5740cd35f448 // indirect
@@ -155,6 +159,7 @@ require (
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect
 	github.com/dlclark/regexp2 v1.7.0 // indirect
 	github.com/dsnet/compress v0.0.2-0.20210315054119-f66993602bf5 // indirect
+	github.com/emersion/go-sasl v0.0.0-20200509203442-7bfe0ed36a21 // indirect
 	github.com/felixge/httpsnoop v1.0.3 // indirect
 	github.com/fxamacker/cbor/v2 v2.4.0 // indirect
 	github.com/go-ap/errors v0.0.0-20221205040414-01c1adfc98ea // indirect