This merges the SubscriptionManager and UnsubscriptionManager into a
single SubscriptionManager implementation that handles both register()
and unregister().  This allows us to use direct class-based adaptation
instead of the more clunky getAdapter() API.  We can also eliminate the
funky _get_workflow() implementation detail.

This has a couple of side-effects.  .confirm() must lookup the token in
the pendings database and pull out the pending type, dispatching to the
proper class depending on the type, or raising a LookupError if the
token is None or there is no pendable associated with the given token.
This feels like an acceptable trade-off.

However, this *also* means that IWorkflowStateManager must lose its
'name' argument in its methods.  That's because we won't actually know
the name until its too late.  Honestly, the name wasn't providing much
value anyway (it was always the subclass's name), so losing that seems
fine too.  The complication here is that the name was a primary key in
the 'workflowstate' table, so we need to add its removal in the database
migration.