Accounted for top offset to keep login container centred. Login container fades in on load.
Tested on a variety of window sizes and appears to be centred on all of them (measured it to make sure).
Now included in `$(window).resize();` to update location of Login
container. The login is now more of a visually 'true' centre, with the offset being more towards the top.
Centring on resize, wasn't in original commit to remove one extra event listener.