Add model for networking journal entry
Implement journal-emptying function and server
Defer network operations

Now the API calls 'node_connect_network' and 'node_detach_network' simply make
a journal entry and return.  This breaks 'project_detach_node' and
'delete_network'---they gain an additional check.  Several tests now
explicitly perform the journal, to mimic the old non-deferred functionality.
Add another abstract model, AnonModel

This one has a 'id' (primary key), but not 'label' (user-visible name).