activity versioning causes break of plugin rendering

the versioning of activities, implemented in !187 (merged) causes issues with plugins that refer to the activity. The CreateTopicButtonPublisher. The djangocms_versioning library apparently is not well tested for such a case. We should move the CMS-functionalities to a dedicated model. The good thing is that this model can then also be used for other models that provide CMS functionalities for their detail pages, such as the Event or the Topic model (see #194)