Make Urdu (ur) recognized as RTL language. Refs #20454

 In #15300, Urdu was added, but not added to the list of RTL locales. But it is.

References: ​https://github.com/django/django/blob/master/django/contrib/admin/locale/ur/LC_MESSAGES/django.po shows if you select bidi text, and ​http://en.wikipedia.org/wiki/Urdu says so.

This is for https://code.djangoproject.com/ticket/20454.