BugFix: Activity app when deleting a user

Changes on Activity model and Left Group Signal

There was an INSERT query taking action when you delete a user and the user is current in a group.

When you delete the user the deletion is in cascade mode, also will delete the user and related objects which will make the user force leave a group.

But when the user leaves a group the signal creates a registry in Activity app inserting the current user we deleted, but the current ID for user reference is no more available, then he fails raise the exception and rollback all db changes.

The pre_delete function should fix the problem.