Fixed #7030 -- Handle extraction of UTF-8 messages from Javascript files.

This leads to more duplicated code in make-messages.py, but this is just a
holdover for the immediate problem until we merge make-messages into
management/.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7473 bcc190cf-cafb-0310-a4f2-bffc1f526a37