refactor(client): Asynchronous rendering!

* Asynchronous can be used to check for prerequisites/errors before printing the template, preventing flashes of content before being fully ready.
* Only show the contents of the new screen once all async operations are complete.
* Add fxa-client.js->isSignedIn to check if a user is signed in with the given token.
* Move fxaClient creation from form.js into base.js so that authentication can be checked.
* Clean up base.js->render and the isSignedIn check.
* Update all the vew tests to expect async rendering.

issue #788