A re-authentication page that's more friendly to the user

...rather than asking them to re-sign-in (they're already signed in), this page explains why they have to re-enter their password, and offers appropriate social-signin links (we only accept social links if they
actually force the user to re-enter their password).

This change doesn't actually point users to this re-authentication page yet, it just makes it available so UX, etc can review the page and try the social-signin-confirmation flows.

https://jira.gutools.co.uk/browse/MEM-403

cc @adamnfish @mchv