Ask users to 'add email address' when logged in

[shevski]

I'd like us to display a one-off banner that prompts users who log into thedatahub.org to go and update & their profile & fill in their email address (if we could do it just for those who don't have an email address, then even better) That is, once they log in, they see a banner which says: Please update your profile [here] and add your email address, so you can receive notifications, be able to reset your password and get updates

Then when we build notifications or if we decide to survey people, we can actually contact them. Email address should be required as standard