Plugin missing fix (#5653)

* Add 'is_installed' property to PluginConfig model

- Allow us to show users which plugins are "outdated"

* Cleanup plugin table

- Display clearly if a plugin is no longer installed

* Fixes for plugin table

* Revert change due to failing CI

InvenTree/plugin/models.py

@@ -104,9 +104,9 @@ class PluginConfig(InvenTree.models.MetadataMixin, models.Model):
         self.plugin: InvenTreePlugin = plugin
 
     def __getstate__(self):
-        """Customize pickeling behaviour."""
+        """Customize pickling behavior."""
         state = super().__getstate__()
-        state.pop("plugin", None)  # plugin cannot be pickelt in some circumstances when used with drf views, remove it (#5408)
+        state.pop("plugin", None)  # plugin cannot be pickled in some circumstances when used with drf views, remove it (#5408)
         return state
 
     def save(self, force_insert=False, force_update=False, *args, **kwargs):
@@ -120,14 +120,23 @@ class PluginConfig(InvenTree.models.MetadataMixin, models.Model):
             self.active = True
 
         if not reload:
-            if (self.active is False and self.__org_active is True) or \
-               (self.active is True and self.__org_active is False):
+            if self.active != self.__org_active:
                 if settings.PLUGIN_TESTING:
                     warnings.warn('A reload was triggered', stacklevel=2)
                 registry.reload_plugins()
 
         return ret
 
+    @admin.display(boolean=True, description=_('Installed'))
+    def is_installed(self) -> bool:
+        """Simple check to determine if this plugin is installed.
+
+        A plugin might not be installed if it has been removed from the system,
+        but the PluginConfig associated with it still exists.
+        """
+
+        return self.plugin is not None
+
     @admin.display(boolean=True, description=_('Sample plugin'))
     def is_sample(self) -> bool:
         """Is this plugin a sample app?"""