Update COPPA to be done on the sign_up screen.

[shane-tomlinson]

• 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

[shane-tomlinson]

A note to whomever tests - In Firefox Nightly at least, two things must happen for the session cookie to go away and the user to be able to sign up again:

1. the tab the attempted to sign in on must be closed.
2. the browser must be closed.

Any deviation from this, and you are booted directly back to the /cannot_create_account screen.

[shane-tomlinson]

Screen shot of the new sign up screen:

[nchapman]

On it.

[nchapman]

@shane-tomlinson It looks like account creation and therefore errors are still happening in a different view. Do you see any problem with us moving all of that into the SignUp view?

[nchapman]

Email address is missing during sign up:

[shane-tomlinson]

@nchapman - moved the fxaClient.signUp portions of the code from /create_account to /signin so the error messages stay on the same screen.

[shane-tomlinson]

@nchapman:

so confused!

[nchapman]

@shane-tomlinson Sorry about that! I didn't have the latest code :(. It seems to be working wonderfully now.

[nchapman]

All seems well. Sign in, Sign up, and age restriction all seem to be working as expected. Great work @shane-tomlinson!