Let Sequel know about the old BillingEvent classes

[fixes #56898040]

The class namespace of this model was changed which broke all of the
Single Table Inheritance class loading because we were keeping the exact
class name in the database as the 'kind' field. This caused rows created
before the switch not to be loaded properly.
Protect the droplets and build pack cache upload and download urls by basic auths.
Remove test pollution from new Billing Event spec