[DO NOT MERGE] Login/Logout

Based on @jongilbraith's [login-spike](https://github.com/moneyadviceservice/frontend/compare/login-spike) though this merely checks for the existence of the session variable set by Devise in ``public_website``.

Waiting on the visual design to be finalised.
Good point. It's beyond the scope of our project but adds a bit of convenience for the future