Django-x509

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- [fix] Use ``self.pk`` instead of ``self.id`` to allow more
flexible override of primary key
- Fixed jQuery init issue on django 2.2

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* `36 <https://github.com/openwisp/django-x509/issues/36>`_:
[requirements] Added support for django 2.1
* `44 <https://github.com/openwisp/django-x509/issues/44>`_:
[models] Improved error message format 44
* `61 <https://github.com/openwisp/django-x509/pull/61>`_:
Bumped supported Django version to 2.2 and Python version to 3.7
* `63 <https://github.com/openwisp/django-x509/pull/63>`_:
[bug] Load model after registration in apps
* Bumped cryptography version to 2.8.0, pyopenssl to 19.0.0

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* [admin] Fixed UI bug that prevented changing Cert and CA
* [requirements] cryptography>=2.3.0,<2.4.0
* [requirements] pyopenssl>=17.5.0,<18.1.0
* `41 <https://github.com/openwisp/django-x509/pull/41>`_:
[requirements] Added support for django 2.1
* [admin] Fixed involuntary permanent modification of field list

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* `24 <https://github.com/openwisp/django-x509/issues/24>`_:
[qa] Added django 2.0 & dropped django 1.10
* `25 <https://github.com/openwisp/django-x509/issues/25>`_:
[admin] Automatically select ``certificate`` and ``private_key`` on click
* `33 <https://github.com/openwisp/django-x509/issues/33>`_:
[models] Added ``organizational_unit_name`` in ``Cert`` and ``Ca``

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* [admin] Removed ``serial_number`` from certificate list

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* [models] Reimplemented serial numbers as UUID integers
* [UX] Import vs New javascript switcher