Make django-markupfield a django-app by adding the models.py and therefore make the locales work.

As said in the README: if you want to use the translation feature for
other languages than English, you have two options:

 1. Add ``'markupfield'`` to your ``INSTALLED_APPS`` and use existing
    translations
 2. Look up the ``msgctxt`` and ``msgid`` for your markup types in
    ``markupfield.markup`` and add the translations to your project
    translations.