Add promise returns for controller methods.

[shangzhel]

I realized there was no way for components to know whether an action had succeeded. This PR adds return values to networked methods that carry the status of HTTP responses. Components can await calls to the controllers and show error indicators on failure.

• IAppContext.login() and IAppContext.logout() now return Promise<ResponseStatus>
• ICard.commit() and ICard.delete() now return Promise<ResponseStatus>
• ResponseStatus is a subset of Response that carries only the values of ok, status and statusText.

[waltervan00]

Assuming that login in AppContext is currently a stub for React to use. Looks fine.

[chomosuke]

Looks good