Django-x509

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

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- [requirements] upgraded pyopenssl to 17.5.0 and cryptography to 2.2.0
- [models] Fixed uncaught exception when imported PEM ``certificate`` or
``private_key`` is invalid

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- temporarily downgraded cryptography and pyopenssl versions to avoid
segmentation faults

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- [models] Avoided possible double insertion in ``Base.save``
- [requirements] pyopenssl>=17.1.0,<17.4.0
- [admin] Fixed preformatted look of certificate and private-key fields
- [models] Allow importing certs with invalid country codes
- [models] Allow importing certificate with empty common name
- [tests] Updated data for import test to fix pyOpenSSL issue
- [models] Renamed ``organization`` field to ``organization_name``

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- [models] added ``digest`` argument to ``CRL.export``
- [requirements] pyopenssl>=17.1.0,<17.2.0

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- [migrations] Updated ``validity_start`` on ``Cert`` model