Switched data-sync-complete event from CDI notification to RemoteCallback

It became clear in the todo list demo that different callsites to coldSync() have different "what to do next" needs.
Could have passed a list of CDI qualifiers to coldSync, but why not just use a callback at this point?
Signup and login are now working properly!

At this point, there is no authorization checking, so anybody can sync anybody else's stuff.
This will be fixed shortly!