Add copy_event function.
Add copy_event function.

* copied mostly from copy_event management command
* saving of coaches & sponsors fixed: we need to fetch them before we
change id of the new_event

patjouk/djangogirls,DjangoGirls/djangogirls,DjangoGirls/djangogirls,patjouk/djangogirls,DjangoGirls/djangogirls,patjouk/djangogirls,patjouk/djangogirls