fix: volunteer reactivation
https://trello.com/c/hlrns1nL/227-exception-when-reactivating-volunteers
the error is because of missing attribute invited_by_type on users, which is required by devise_invitable (GitHub - scambra/devise_invitable: An invitation strategy for devise) , there is also a problem in reactivate! function on volunteer, function parameter is mixed with instance method (parameter user mixed with instance method user, same name)