Django-post-office

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* Support for Django 4.0. Thanks domdinicola!
* `cleanup_mail` now deletes emails in batches, which is much nicer to DB when deleting millions of emails. Thanks stevemc4!
* Failure to send an email are now logged as an exception. Thanks SaturnFromTitan!
* Added `es` locale. Thanks ahmontero!
* Fixed admin template discovery issue for case-sensitive filesystems. Thanks fasih!
* Fixes: `SMTPServerDisconnected` error. Thanks weimens!
* Various maintenance work by jrief and mogost.

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* Fixed an issue with Celery integration that could cause duplicate emails. Thanks jrief!

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* Fixed an issue where Post Office's admin interface doesn't show. Thanks christianciu!

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* Added missing migration file (some model fields now offer a help text).

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* Added the capability to configure retries via `MAX_RETRIES` and `RETRY_INTERVAL` configuration settings. Thanks Houtmann and jrief!
* The admin interface has been improved to show previews of emails. If you want HTML emails to be rendered,
please install `bleach`. Thanks jrief!
* Add `Message-ID` to the Email model. This allows administrators to trace back emails. Thanks jrief!
* Added `CELERY_ENABLED` settings. Thanks elineda!
* Fixes an issue that prevents PDS attachments from being sent. Thanks patroqueeet!

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* Allow `tasks.py` to be imported when Celery is not installed. This allows
auto-discovery by other task systems such as Huey to succeed.
* Fix duplicate key problem in template editor in Django admin.