Move display of LogEntry details from the `logentry_display` and
`logentry_object_link` signals to a class hierarchy based approach.
For each action_type, an instance of a subclass of `LogEntryType`
is registered in the `log_entry_types` registry.
Analogous to EventPluginSignal, this registry is an `EventPluginRegistry`,
so it keeps track of the plugin the LogEntryType is defined in.
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Co-authored-by: Raphael Michel <michel@rami.io>
Co-authored-by: Richard Schreiber <schreiber@rami.io>
* Upgrade Django to 3.0 and other dependencies to recent versions
* Fix otp version contsraint
* Remove six dependency
* Resolve some warnings
* Fix failing tests
* Update django-countries
* Resolve all RemovedInDjango31Warnings in test suite
* Run isort
* Fix import
* Update PostgreSQL version on travis
