Remove GroupActivity model and implement ChangeLoggedModel model instead

[jokiefer]

Status quo

There is a GroupActivity model to show group based activities. All GroupActivity objects are generated by hard coded lines somewhere in the code. This is bad practice, cause there are many depending codelines.

Enhancement

Take a look on the netbox solution to track changes of models. There is one ChangeLoggedModel as an abstract model, which has all needed logic to store ObjectChange objects on model instance changes. This is a high generic way to track history of a instance of a model. We should implement the same, instead of GroupActivity.