Add handling for message reply via email

If the reply_email_domain configuration is set the messages plugin will
use unique Reply-To addresses for every email. They contain enough
information to reconstruct the message context and a hash to avoid
user forgery and spam.
A mail transfer agent must be configured to call the new rake task
foodsoft:parse_reply_email for incoming mails. The rake task requires
the receipt of the mail in the RECIPIENT variable and the raw message
via standard input. An example invocation would look like:
rake foodsoft:parse_reply_email RECIPIENT=f.1.1.HASH < test.eml

plugins/messages/app/mailers/messages_mailer.rb

@@ -4,9 +4,17 @@ class MessagesMailer < Mailer
     set_foodcoop_scope
     @message = message
 
+    reply_email_domain = FoodsoftConfig[:reply_email_domain]
+    if reply_email_domain
+      hash = message.mail_hash_for_user recipient
+      reply_to = "Foodsoft <#{FoodsoftConfig.scope}.#{message.id}.#{recipient.id}.#{hash}@#{reply_email_domain}>"
+    else
+      reply_to = "#{show_user(message.sender)} <#{message.sender.email}>"
+    end
+
     mail subject: "[#{FoodsoftConfig[:name]}] " + message.subject,
          to: recipient.email,
          from: "#{show_user(message.sender)} via Foodsoft <#{FoodsoftConfig[:email_sender]}>",
-         reply_to: "#{show_user(message.sender)} <#{message.sender.email}>"
+         reply_to: reply_to
   end
 end