This patch completes the pipe of creating a dummy account using
RxSwift and MVVM.

The AccountsService now exposes a public shared responseStream that
observers can listen to.
Callers will call this service's methods directly and wait for any
interesting response on the stream.

Tuleap: #1391
Change-Id: I2eb5abac9eb012892a806ee2fb73df8730edba87