Refactor: BaseInvenTreeSetting (#4834)

* Added typing for settings

* Refactored common.BaseInvenTreeSetting model to make it more generic

* Use older syntax for union types

* Added protected option to typing

* Remove now unused code

* Remove old 'get_kwargs' method as it is replaced by 'get_filters_for_instance'

* Trigger ci

InvenTree/plugin/base/integration/SettingsMixin.py

@@ -1,14 +1,25 @@
 """Plugin mixin class for SettingsMixin."""
 import logging
+from typing import TYPE_CHECKING, Dict
 
 from django.db.utils import OperationalError, ProgrammingError
 
 logger = logging.getLogger('inventree')
 
+# import only for typechecking, otherwise this throws a model is unready error
+if TYPE_CHECKING:
+    from common.models import SettingsKeyType
+else:
+    class SettingsKeyType:
+        """Dummy class, so that python throws no error"""
+        pass
+
 
 class SettingsMixin:
     """Mixin that enables global settings for the plugin."""
 
+    SETTINGS: Dict[str, SettingsKeyType] = {}
+
     class MixinMeta:
         """Meta for mixin."""
         MIXIN_NAME = 'Settings'
@@ -56,7 +67,7 @@ class SettingsMixin:
         """
         from plugin.models import PluginSetting
 
-        return PluginSetting.get_setting(key, plugin=self, cache=cache)
+        return PluginSetting.get_setting(key, plugin=self.plugin_config(), cache=cache)
 
     def set_setting(self, key, value, user=None):
         """Set plugin setting value by key."""