Django-x509

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- [deps] Updated cryptography minimum version to 3.2 for security reasons
The version range has also been relaxed to avoid having to update it too
often, future versions up to but excluding 4.0 will be accepted
- [fix] Removed ``static()`` call from admin media
- [tests] Updated openwisp-utils[qa] to 0.7

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Changes
~~~~~~~

- Models: updated ``max_length`` of ``common_name`` (``Ca`` and ``Cert``)
to from ``63`` to ``64``, following conventions about maximum length of
common names and hostnames

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Features
~~~~~~~~

- Added swappable models, improved extensibility
- Improved documentation on `how to extend django-x509
<https://github.com/openwisp/django-x509#extending-django-x509>`_

Changes
~~~~~~~

- **Breaking change**: systems using django-x509 as a library must set
``DJANGO_X509_CA_MODEL`` & ``DJANGO_X509_CERT_MODEL`` values in their
settings.py when upgrading or an exception like the following one will
be raised:

``django.core.exceptions.ImproperlyConfigured: Could not find
django_x509.Ca!``

- Added support for django 3.1
- Added support for cryptography 3.0.0

Bugfixes
~~~~~~~~

N/A

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- Added possibility to renew CAs and certificates
- Updated dependency to support cryptography 2.9

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- Switched back to jsonfield

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- Ensured RFC5280 datetime standard
- Increased maximum length of x509 serial number to 48
- jsonfield2 version set to >=3.1.0,<4.0.0