Add support for processing incoming mail messages in reply to message

notifications and diary comment notifications, turning the maill messages
into reply messages through the site messaging system.

app/models/diary_comment.rb

@@ -4,4 +4,13 @@ class DiaryComment < ActiveRecord::Base
 
   validates_presence_of :body
   validates_associated :diary_entry
+
+  def digest
+    md5 = Digest::MD5.new
+    md5 << diary_entry_id.to_s
+    md5 << user_id.to_s
+    md5 << created_at.xmlschema
+    md5 << body
+    md5.hexdigest
+  end
 end

app/models/message.rb

@@ -9,4 +9,14 @@ class Message < ActiveRecord::Base
   validates_inclusion_of :message_read, :in => [ true, false ]
   validates_associated :sender, :recipient
   validates_as_utf8 :title
+
+  def digest
+    md5 = Digest::MD5.new
+    md5 << from_user_id.to_s
+    md5 << to_user_id.to_s
+    md5 << sent_on.xmlschema
+    md5 << title
+    md5 << body
+    md5.hexdigest
+  end
 end

app/models/notifier.rb

@@ -47,6 +47,7 @@ class Notifier < ActionMailer::Base
   
   def message_notification(message)
     common_headers message.recipient
+    from_header message.sender.display_name, "m", message.id, message.digest
     subject I18n.t('notifier.message_notification.subject', :user => message.sender.display_name, :locale => locale)
     body :to_user => message.recipient.display_name,
          :from_user => message.sender.display_name,
@@ -62,6 +63,7 @@ class Notifier < ActionMailer::Base
 
   def diary_comment_notification(comment)
     common_headers comment.diary_entry.user
+    from_header comment.user.display_name, "c", comment.id, comment.digest
     subject I18n.t('notifier.diary_comment_notification.subject', :user => comment.user.display_name, :locale => locale)
     body :to_user => comment.diary_entry.user.display_name,
          :from_user => comment.user.display_name,
@@ -103,8 +105,14 @@ private
   def common_headers(recipient)
     recipients recipient.email
     locale recipient.preferred_language_from(I18n.available_locales)
-    from "webmaster@openstreetmap.org"
+    from "OpenStreetMap <webmaster@openstreetmap.org>"
     headers "return-path" => "bounces@openstreetmap.org",
             "Auto-Submitted" => "auto-generated"
   end
+
+  def from_header(name, type, id, digest)
+    if domain = APP_CONFIG['messages_domain']
+      from "#{name} <#{type}-#{id}-#{digest[0,6]}@#{domain}>"
+    end
+  end
 end