Django-post-office

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* Added compatibility with Django 2.0. Thanks PreActionTech and PetrDlouhy!
* Added natural key support to `EmailTemplate` model. Thanks maximlomakin!

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* Fixed memory leak when multiprocessing is used.
* Fixed a possible error when adding a new email from Django admin. Thanks ivlevdenis!

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* `_send_bulk` now properly catches exceptions when preparing email messages.

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* Fixed an infinite loop bug in `send_queued_mail` management command.

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* `_send_bulk` now allows each process to use multiple threads to send emails.
* Added support for mimetypes in email attachments. Thanks clickonchris!
* An `EmailTemplate` can now be used as defaults multiple times in one language. Thanks sac7e!
* `send_queued_mail` management command will now check whether there are more queued emails to be sent before exiting.
* Drop support for Django < 1.8. Thanks fendyh!

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* Django 1.10 compatibility fixes. Thanks hockeybuggy!
* Fixed an issue where Django would sometimes create migration files for post-office. Thanks fizista!