Update COPPA to be done on the sign_up screen.

* Use a session cookie to keep track of whether the user is too young.
* If the user revisits the signup page after being booted out for being too young, ship them back to the "cannot_create_account" page.
* Update the text on the Cannot Create Account screen to go along with @jgruen's new mocks and legal's feedback.

issue #226
Move the call to FxaClient.signUp to the sign_up view.

* Delete the old interstitial screen.
* Simplify!
Update the unit tests to handle the asynchronous sign up.