Carve up app_state.dart

This file is large an unwieldy. Next time we don't have any significant pending merge requests we should take a stab at carving this file up. I'd like to see a state directory containing files like uploading.dart, downloading.dart, upgrading.dart, portal.dart, tasks.dart, ops.dart, events.dart, deploy.dart, and probably others... and then one top level app_state.dart that consumes them.

To start this should be done w/o making any significant changes to the implementations and classes themselves, just moving them. Once classes/types are in their new files and this is merged we can look into creating clearer abstractions and refactoring.