Force builtin plugins to be active (#4276)

* Force builtin plugins to be active

- Not setting the "active" flag can cause ambiguous errors
- e.g. some settings don't work if the plugin is inactive
- By design, builtin plugins are always "active" anyway
- This PR just forces the active field to True

* Fix unit tests

InvenTree/plugin/models.py

@@ -148,6 +148,10 @@ class PluginConfig(models.Model):
 
         ret = super().save(force_insert, force_update, *args, **kwargs)
 
+        if self.is_builtin():
+            # Force active if builtin
+            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):