Django-parler

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* Fixed Django 2.0 ``contribute_to_class()`` call signature.
* Fixed "Save and add another" button redirect when using different admin sites.
* Fixed import location of ``mark_safe()``.

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* Fixed HTML output in Django 2.0 for the the ``language_column`` and ``all_languages_column`` fields in the Django admin.

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* Added Django 2.0 support.
* Fixed ``get_or_create()`` call when no defaults are given.

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* Fixed checkes for missing fallback languages (``IsMissing`` sentinel value leaked through caching)
* Fixed preserving the language tab in the admin.
* Fixed ``get_or_create()`` call.

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* Dropped Django 1.5, 1.6 and Python 2.6 support.
* Fixed Django 1.10 / 1.11 support:

* Fix ``.language('xx').get()`` usage.
* Fix models construction via ``Model(**kwargs)``.
* Fix test warnings due to tests corrupting the app registry.

* Fix support for ``ModelFormMixin.fields`` in ``TranslatableUpdateView``.
Django allows that attribute as alternative to setting a ``form_class`` manually.

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* Added ``delete_translation()`` API.
* Added ``PARLER_DEFAULT_ACTIVATE`` setting, which allows to display translated texts in the default
language even through ``translation.activate()`` is not called yet.
* Improve language code validation in forms, allows to enter a language variant.
* Fixed not creating translations when default values were filled in.
* Fixed breadcrumb errors in delete translation view when using django-polymorphic-tree_.