Nav signin

In this commit I created new signin routes which implement OAuth 2.0 single sign-on using the Windows Live REST API rather than the JavaScript API which we were using previously. To use the JavaScript API, the following external script had to be loaded from the `js.live.net` server in order to sign users in.

    js.live.net/v5.0/wl.js

Using the Windows Live REST API over the JavaScript API has the following advantages:

 - Does not have to load the external Windows Live script from the external host `js.live.net` in order to sign users in.

 - Windows Live login page does not open in a pop-up window.

 - Does not use AJAX to send user data to the server.

 - Completely eliminates the need for a signin controller script.

**Changelog:**

 - Removed route `signin.post`, kept route `signin.get`, added routes `signin.authenticate` and `signin.success` in `server/router/index.js`.

 - Added route `signout` in `server.router/index.js`.

 - Added new signin paths `/signin/authenticate` and `/signin/success` to the open paths in `server/middleware/wlauth.js`.

 - Extracted the navbar markup into its own file `navbar.html` which is included in the layout `default.html`.

 - Added new navbar markup in `navbar.html`

 - Added custom navbar styles in `public/css/styles.css`

 - Added new signin page markup in `server/views/signin.html`.

 - Made the `home` route redirect to `staff/staffHomePage` for staff.

This external script does not need to be loaded at all anymore. We also do not
have to make a seperate AJAX requet to send user data to the server.