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
Test classes rather than equality in billing events

This works around MySQL and PostgreSQL adapters claiming that an object
is not in a collection even if all of the attributes are the same.
Fix the wrong droplet keys get generated.
Renamed key_from_sha1 to partitioned_key
Moving things around in blobstore test. 
Gradually remove the use of blobstore.files as it should not be exposed.