refactor handler.ts Handlers

[alee]

Initial draft design of interacting components:

• API knows the endpoint and how to invoke it and with what parameters and drives the process
• ServerResponseHandler builds coherent data structures out of a server response for use by the Component
• Components are responsible for stitching together the ServerResponseHandler and doing stuff based on its state (dismiss a modal, redirect on success, display a message somewhere on the DOM, display validation errors)