Missing authorization check in event quick setup view

The "quick setup" view presented to users after they first create an
event allows to set up the most critical parts of an event in just a few
clicks. This view did not properly check that the user has permission
to change configuration for the given event. An attacker could use a
well-timed request to create products, quotas, set bank transfer
configuration, or connect a stripe account to an event they do not have
access to.