Babel allows us to register babel extractors via entry points.
This means we no longer need to have extractor definitions in
each babel config files. All horizon plugins have copies of
babel-django.cfg and babel-djangojs.cfg now. By dropping the
extractor defintiions from babel config files, we can make
a potential migration from django-babel to enmerkar (and
we can also avoid more modifications of babel config files
even if more transition is required).
django-babel and enmerkar both define 'django' extractor
in the entry point, so we can drop it from babel-django.cfg.
This commit also registers 'angular' extractor, so we can drop
the extractor for AngularJS from babel-djangojs.cfg.