Identity: guide users to sign in to resend their verification email

The 'Resend my verification email' button did nothing if you were NOT
signed in, see the button on this page:

https://profile.theguardian.com/verify-email/useThisBrokenToken

Now the user is guided to sign-in instead, and this action automatically
resends their validation email.

https://jira.gutools.co.uk/browse/IDN-1736