Use the message title as the subject of notification mails

Make message notification mails use the message title as the subject
so that it is properly perserved when replying by email. Closes #3051.

app/models/notifier.rb

@@ -48,7 +48,7 @@ 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)
+    subject message.title
     body :to_user => message.recipient.display_name,
          :from_user => message.sender.display_name,
          :body => message.body,

config/locales/en.yml

@@ -1062,7 +1062,6 @@ en:
       header: "{{from_user}} has commented on your recent OpenStreetMap diary entry with the subject {{subject}}:"
       footer: "You can also read the comment at {{readurl}} and you can comment at {{commenturl}} or reply at {{replyurl}}"
     message_notification:
-      subject: "[OpenStreetMap] {{user}} sent you a new message"
       hi: "Hi {{to_user}},"
       header: "{{from_user}} has sent you a message through OpenStreetMap with the subject {{subject}}:"
       footer1: "You can also read the message at {{readurl}}"