Layout: Use a single layout component

[drewblaisdell]

Eventually, the following will be true in Calypso:

• It will be possible to sign up from within the desktop app.
• Users will be logged in at the end of the signup flow without being redirected through wp-login.php.

Right now, we use separate layout components for when the user is logged in/out or on the login screen. To make it possible to transition between these states without wiping out #content, we should use a single layout component.

Much of the work for this is already exists in #2813.

cc @scruffian