[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.
Given we have a `user_signed_in?` helper and the strings "Sign in", "Sign out" maybe its worth using "sign_in / sign_out" across the board over "login / logout"?