Django-yubin

Fixed:
* Race condition between Celery and database transactions (https://github.com/APSL/django-yubin/pull/74)

Changed:
* Improve performance of data migration when migrating from versions < 2.0 (https://github.com/APSL/django-yubin/pull/69)

Fixed:
* Perform unfolding of headers when parsing messages (https://github.com/APSL/django-yubin/pull/71)

* Ensure parsed e-mail message doesn't discard information (https://github.com/APSL/django-yubin/pull/67).

* Send and queue emails with Celery instead of with Cron.
* Drop priority headers (useless with queues).
* Storage backends to save emails in databases, file storages, etc.
* Cc and Bcc support.
* Supported versions: Python 3.8~3.11, Django 3.2~4.2, Celery 5.0~5.2.
* Migrate CI/CD from Travis to Github Actions.
* Docker Compose for external dependencies in development environment.
* Get django_yubin version programmatically.
* Update docs.

Remove abandoned pyzmail36 dependency with mail-parser

Add optional `MAILER_MESSAGE_SEARCH_FIELDS` setting. It's a tuple of strings with the fields to use in `admin.Message.search_fields` attribute.