Add PIN input screen

[dllewellyn]

Requires

• A global object that can be called to ask 'is the user logged in'
• A global object to log a user out
• A way of creating a PIN for the first time
• A way of storing said pin securely
• A way of verifying the PIN for future
• A callback that can be used to register for pin changes (i.e. has the user logged out)