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).

https://jira.gutools.co.uk/browse/MEM-403
Merge pull request #6437 from guardian/reauthentication-page

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