[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.
Yep, per the huddle we'll be dealing with combinations of the following links:

* sign in
* sign out
* my account
* register