Django-addendum

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* Drop Python 2 and Django 1.11 support
* Add Django 2, 3 support

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* Fix issue in which variables may be explicitly boolean in render method

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* Fix a packaging error

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* Removes South support!
* Django 1.8 and 1.11 support only for now
* MAJOR: updates behavior for templated snippets to prevent template errors
from bubbling up.

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* Drop support for Django 1.4
* Packaging fix
* Automatically create missing Snippet instances
* Bug fixes for cache invalidation, admin langauges, text autoescaping

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* Fixes migration issue (possibly breaking for some users): the field choices
as provided in the SnippetTranslation model populated a new migration when
installed in a project, and these noticeably changed after an update to the
project's testing framework. This was 'fixed' by removing the choices from
the model to a ModelForm class, but users with 0.3.4 installed might have an
extra migration. Only affects Django 1.8+.