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.

fixes #265 
fixes #259