Django-netjsonconfig

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- [feature] Added signals: ``config_status_changed``, ``checksum_requested``, ``config_download_requested``
- [feature] Added the possibility of specifying default values for variables used in templates
- [change] **Backward incompatible change**: the ``config_modified`` signal is not emitted anymore when the device is
created
- [change] VPN files now have 0600 permissions by default
- [change] Increased minimum `netjsonconfig <https://github.com/openwisp/netjsonconfig>`_ version to 0.8.0
- [fix] Avoid errors being hidden by tabs
- [fix] Fixed clashes between javascript schema validation and variables

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- Added controller view that allows to update the device information (firmware version used)
- Added ``NETJSONCONFIG_HARDWARE_ID_AS_NAME`` setting

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- [admin] Fixed 500 internal server error in change device (now returns 404 correctly)

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- [controller] Added ``NETJSONCONFIG_REGISTRATION_SELF_CREATION``
- [models] Handled accidental duplication of files across templates
- [controller] Update hardware device info during registration
(if the device already exists, the registration will update its info)
- [admin] Moved ``hardware_id`` field in device list admin
- [bugfix] Fixed broken preview when using ``hardware_id`` context var
- [admin] Hidden device configuration context field into advanced options
- [models] Removed LEDE from the OpenWRT backend label

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- [models] Improved consistent key generation, now a consisten key is generated
also when creating devices from the admin interface (or via model API),
before it was only done during registration
- [admin] Fixed unsaved changes JS bug that was triggered in certain cases
- [deps] Switched back to jsonfield

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- Dropped django-x509<0.6.1 support
- Dropped django-reversion<=3.0.5 support
- [admin] Fixed minor bug in VPN client template config editor