Replace mimetype with content_type in HttpResponses (bug 1020943)

https://bugzilla.mozilla.org/show_bug.cgi?id=1020943

`mimetype` is deprecated: https://docs.djangoproject.com/en/1.6/ref/request-response/#django.http.HttpResponse.__init__
I do too, but here with the new keyword argument it doesn't fit in 80 chars :(