Add "Reply to this email directly" to notification footer.

app/views/layouts/notify.html.haml

@@ -36,6 +36,10 @@
         —
         %br
         - if @target_url
+          - if @reply_by_email
+            Reply to this email directly or
+            #{link_to "view it on GitLab", @target_url}.
+          - else
             #{link_to "View it on GitLab", @target_url}
           = email_action @target_url
         - if @project && !@disable_footer