During the admin UI rewrite, I forgot a parameter to ngettext(). This

previously went unnoticed, but with our new babel-plugin-django-gettext,

it now becomes a build-time error.
This change fixes this up to provide the third argument, the count, and

at the same time switches to using the new syntax, simplifying the code

and reducing the setup for it.

Verified the generated code (when used against an upcoming build of

babel-plugin-django-gettext, which fixes ngettext-related issues).