do not translate canned responses for editor reviews (bug 979484)

Fix [bug 979484](https://bugzilla.mozilla.org/show_bug.cgi?id=979484)

The previous PR #23 was incomplete: the canned responses were still translated.
This is now fixed: the canned response (the ```reason``` in the context) is used in ```send_mail_jinja``` which wasn't in the overriden language part. It's now indented properly (and the test has been modified to validate this).