Set all polymorphic associations of Activity as optional
Set all polymorphic associations of Activity as optional

* Admin invitations don't have site or subject
* There are a lot of activities performed by automated tasks like
calendars synchronizations or budgets updates with blank author
* Most of activities have blank recipient

PopulateTools/gobierto-dev,PopulateTools/gobierto,PopulateTools/gobierto,PopulateTools/gobierto-dev,PopulateTools/gobierto,PopulateTools/gobierto,PopulateTools/gobierto-dev,PopulateTools/gobierto-dev