Logout link added

Logout link added to left of version no, next to logo in top right
Clicking this triggers the logout function, which directs the browser to
the same window location with ?logout QS
The settings.php file is run again (via inclusion) and finding the QS
var, sets the loggedIn var set to false (as a double measure to the
session also being destroyed) and header location boots user back to
login screen