Improve and finish the fallback to the In-Reply-To and References header for the reply-by-email feature

A few things to note:
- The IncomingEmail feature is now enabled even without a
  correctly-formatted sub-address
- Message-ID for new thread mail are kept the same so that subsequent
  notifications to this thread are grouped in the thread by the email
  service that receives the notification
  (i.e. In-Reply-To of the answer == Message-ID of the first thread message)
- To maximize our chance to be able to retrieve the reply key, we look
  for it in the In-Reply-To header and the References header
- The pattern for the fallback reply message id is "reply-[key]@[gitlab_host]"
- Improve docs thanks to Axil

2 files changed, 4 insertions, 4 deletions

diff --git a/spec/fixtures/emails/key_in_headers_reply.eml b/spec/fixtures/emails/reply_without_subaddressing_and_key_inside_references.eml
index 8f288775d0a..39d5cefbc2a 100644
--- a/spec/fixtures/emails/key_in_headers_reply.eml
+++ b/spec/fixtures/emails/reply_without_subaddressing_and_key_inside_references.eml
@@ -3,12 +3,12 @@ Received: from iceking.adventuretime.ooo ([unix socket]) by iceking (Cyrus v2.2.
 Received: from mail-ie0-x234.google.com (mail-ie0-x234.google.com [IPv6:2607:f8b0:4001:c03::234]) by iceking.adventuretime.ooo (8.14.3/8.14.3/Debian-9.4) with ESMTP id r5DL3nFJ016967 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>; Thu, 13 Jun 2013 17:03:50 -0400
 Received: by mail-ie0-f180.google.com with SMTP id f4so21977375iea.25 for <reply@appmail.adventuretime.ooo>; Thu, 13 Jun 2013 14:03:48 -0700
 Received: by 10.0.0.1 with HTTP; Thu, 13 Jun 2013 14:03:48 -0700
-In-Reply-To: <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>
-References: <issue_1@localhost> <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>
 Date: Thu, 13 Jun 2013 17:03:48 -0400
 From: Jake the Dog <jake@adventuretime.ooo>
 To: reply@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
+In-Reply-To: <issue_1@localhost>
+References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;
diff --git a/spec/fixtures/emails/valid_reply.eml b/spec/fixtures/emails/valid_reply.eml
index e2b974174bd..980e10a8812 100644
--- a/spec/fixtures/emails/valid_reply.eml
+++ b/spec/fixtures/emails/valid_reply.eml
@@ -3,12 +3,12 @@ Received: from iceking.adventuretime.ooo ([unix socket]) by iceking (Cyrus v2.2.
 Received: from mail-ie0-x234.google.com (mail-ie0-x234.google.com [IPv6:2607:f8b0:4001:c03::234]) by iceking.adventuretime.ooo (8.14.3/8.14.3/Debian-9.4) with ESMTP id r5DL3nFJ016967 (version=TLSv1/SSLv3 cipher=RC4-SHA bits=128 verify=NOT) for <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>; Thu, 13 Jun 2013 17:03:50 -0400
 Received: by mail-ie0-f180.google.com with SMTP id f4so21977375iea.25 for <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>; Thu, 13 Jun 2013 14:03:48 -0700
 Received: by 10.0.0.1 with HTTP; Thu, 13 Jun 2013 14:03:48 -0700
-In-Reply-To: <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>
-References: <issue_1@localhost> <reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo>
 Date: Thu, 13 Jun 2013 17:03:48 -0400
 From: Jake the Dog <jake@adventuretime.ooo>
 To: reply+59d8df8370b7e95c5a49fbf86aeb2c93@appmail.adventuretime.ooo
 Message-ID: <CADkmRc+rNGAGGbV2iE5p918UVy4UyJqVcXRO2=otppgzduJSg@mail.gmail.com>
+In-Reply-To: <issue_1@localhost>
+References: <issue_1@localhost> <reply-59d8df8370b7e95c5a49fbf86aeb2c93@localhost>
 Subject: re: [Discourse Meta] eviltrout posted in 'Adventure Time Sux'
 Mime-Version: 1.0
 Content-Type: text/plain;