[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.
`when the user is not signed in` ?
`when the user is signed out` ?
`when the user is anonymous` ?