Add ability to permit staff and/or superusers by default

Add new constructor args to PermissionsRegistry: allow_staff,
allow_superuser, allow_anonymous. These all set the default that is
used when the corresponding arg isn't passed to
PermissionsRegistry.registry(). When the first two are set via the
constructor, then all decorated views will allow staff and  superusers
access by default (and the main permission logic won't be invoked,
which could be a big performance boost in some cases).