More private password resets, and account linking for Google sign-in

The forgot-password form now gives the same answer whether or not an email address has an account, so it can't be used to find out who uses CompleteStatus.

We've also built account linking for Google sign-in, which isn't available yet. Once the "Continue with Google" button appears on the sign-in page, an account you created with a password can be linked by signing in with your password once; after that Google signs you in. Until then, sign in with your email and password as usual.