Added new events to allow 3rd parties to interact with Activity logging

Events will simplify interaction with the recording of Activities.

* Added firing of event BeforeProcessingActivityNotifications in ActivityModel::Save(). This was necessary for a post scheduling feature to be implemented, to prevent emails from being sent until the post is actually published.
* Added firing of event AfterActivityQuery in ActivityModel::GetWhere(). This allows to filter activities that should not be displayed, based on any criteria.