Add Loading Spinner As Login is Processed

[RaniSputnik]

When the client begins to query the server it would be good to add some visual feedback on the browser page to indicate that something is happening. This would be especially effective in a cross-device scenario where you can see loading bars on both devices as they appear to "sync up".

This would be most effective for CPS logins, or if websockets are used for pushing authentication state updates to the browser as described in #21.

We could do this by using three states for the paging endpoint / websocket messages eg.

404 - Not Found // No session in progress
202 - Accepted // Client authentication in progress
302 - Found // Authentication complete, redirect the browser

This should be considered an enhancement to #20