This means that we don't create a spurious MatrixClient which is thrown away by
the SessionLoader (whilst still ensuring that the rehydrated matrixclient
follows the same code path as matrixclients created at other points in the
session load process).
Take some of the magic out of MatrixChat.componentDidMount() into a new
component.
Also delete the MatrixChat test. It wasn't really doing much, is broken by the
change, and I am replacing it with (better) app-level tests in the vector
project.
Also try to refactor some of the login/logout code out of MatrixChat and into a separate Lifecycle.js. This still isn't great, but it at least gets some code out of MatrixClient.