Django-flatpages-i18n

* Complete refactoring of whole package.
* Dropped support for Python 2
* django-modeltrans replaced django-modeltranslations
* flatpage doesn't have parent relationship
* dropped middleware, using urls instead
* dropped machine name for menu items
* dropped support for redactor editor
* GIN indexes
* WARNING: migrations are not compatible with versions < 1.0.0 !

* support for languages with hyphens

* support for Django 1.11

* language code added to absolute path of FlatPage_i18n model
* support for django-seo2 app

* target of menu items

* translation fields are synced automatically after initial migration